Behind one unit the soil has eroded so far a young resident is now extremely worried about her son playing on the back patio.

"It has just been gradually breaking off closer and closer to the balcony," said the mother, who asked not to be named.

"The rent is being reduced because it is basically a sinkhole at the minute. They were fixing the retaining wall.

"But, gradually every week it just gets worse and it is now just become a big hole."

She pays a share of $500 a week for her three-bedroom unit, at Elanora, near Palm Beach.

Palm Beach/Elanora retaining walls. Photo: Tony Moore

Police investigating fraud and collapsing retaining walls in SEQ

This collapsed retaining wall behind the units is one of a number of dodgy retaining walls, sinkholes, large cracks in carports and concrete walls around south-east Queensland, along with 300 allegedly suspect certification documents for retaining walls.

The documents are under investigation by fraud police.

Police are now investigating allegations one business copied the signature of an engineer working for another business on to their different retaining wall documents.

Housing Minister Mick de Brenni was shown questionable certification documents in November 2016 and asked to investigate.

Queensland's two building watchdogs - the Board of Professional Engineers and the Queensland Building and Construction Commission - began to investigate building faults in March 2017.

Jon Gormley, a principal legal officer at the Board of Professional Engineers of Queensland, confirmed this at a Civil Contractors Federation meeting in Brisbane on June 1.

"The other issue is there have been a number of allegations of allegedly fraudulent Form 16s," Mr Gormley told the lunch.

In construction work the Form 16 shows a building certifier or a civil engineer has inspected the building work and certifies it is satisfactory.

"And what I mean by that is, that the Form 16s have a name, a title and a signature which has been digitally added," he said.

"And the allegation by the registered professional engineer is that they had no professional association with that project, (they) haven't authorised it and (they) haven't issued that Form 16."

Case one: 30 Sapphire Court, Elanora at Palm Beach

Dan Russell says the retaining wall below his units collapsed in July 2016, again in January 2017 and worsened after Cyclone Debbie's heavy rains in late March.

Retaining walls near Palm Beach. Photo: Tony Moore

In May a meeting was held onsite between Mr Russell, Gold Coast City Council, the Queensland Building and Construction Commission and a representative from the company that built the retaining wall.

Mr Russell says he paid $478,000 in infrastructure fees to Gold Coast City Council and a further $20,000 for a stormwater drain.

He now questions if that money was well spent.

The retaining wall has collapsed in two places, cement beams and timber left strewn on the ground, while the soil has slipped two metres towards a creek about 50 metres below the unit complex. That creek is now blocked.

The erosion has slipped so badly it is right up against the wall of one unit.

Mr Russell believes little is being done to fix the problems faced by the people living in the units if the soil slips continue.

"They have been patching it for two years," Mr Russell said.

"From my point of view, the work that has been done there is just band-aiding the problem, without fixing it."

"Are they just going to wait until my building falls over?"

What does Gold Coast City Council say?

Gold Coast City Council said it was conducting an "extensive investigation" into the retaining wall collapse, which it estimates could cost $1 million to repair.

The council is getting geotechnical advice into the wall collapse, it said in a statement.

"We are unable to comment any further at this stage except to say that council does not believe at present that it is responsible for the landslip and is taking action to minimise damage."

A spokeswoman says the Board of Professional Engineers is investigating whether there are problems with the certification documents in the retaining wall work.

"The application was part of the developer's Operational Works and Council and the Board of Professional Engineers are investigating this," she said.

Repairs have to wait until a solution is designed by a geotechnical engineer.

Soil tests run by the council show the soil in the area is classified as "uncontrolled fill", requiring retaining wall piers to be sunk more deeply than traditional retaining wall piers. That is being investigated by the Board of Professional Engineers and Gold Coast City Council.

Case two: 17 York Place, Oxley and Rhonda Place, Oxley

One Oxley homeowner, Ahmad Dehbozorgi, of 17 York Place, says his collapsed retaining wall has forced big cracks in his garage floor.

Damaged carport in Oxley. Photo: Tony Moore

"It is just not done properly," Mr Dehbozorgi says.

"It is not holding the soil. It is not holding the infrastructure properly.

"I think the material that they used is not good. They haven't used the right material for this purpose."

A fellow Oxley resident, Adam McGourty of nearby Rhonda Place, told of cracks and sinkholes appearing in his backyard and soil sinking through his retaining wall.

Nearby, Frank Rosa, also in Rhonda Place, says that when his family moved in 10 years ago the concrete retaining walls were OK.

"Then I started to notice them beginning to crack. And not just around our place, but around the neighbours as well," he says.

"Ours shifted forward as well."

He told Fairfax Media he has spent $20,000 to underpin his Oxley house.

What does Brisbane City Council say?

In late April the Brisbane City Council promised to repair Mr Dehbozorgi's wall and began to investigate retaining walls in the rest of the Oxley residential estate, after inquiries from Fairfax Media.

On May 10, Fairfax Media was told the Brisbane City Council was contacting the estate's developer because the council discovered many other walls in the estate were "in poor condition".

Some have simply collapsed.

Damaged concrete retaining walls in Oxley. Photo: Tony Moore

Others walls show rusting reinforcing rods inside the concrete walls, while other no longer hold together and barely "retain" anything.

Damaged concrete retaining walls in Oxley. Photo: Tony Moore

"It appeared that these walls were constructed as part of a development constructed 13 years ago," the council statement reads.

On June 2, Fairfax Media was told the developer would be asked to pay to repair the poor retaining walls at the estate.

"Council intends to seek restitution from the developer for the cost of repairs, however, further investigations into how and why this structural failure has occurred are still under way.

"It is critical that council understands whether there are further structural issues with the site prior to seeking funds for repairs to the retaining wall.

"Council is also seeking independent analysis of the materials used in the retaining wall to determine if these were the cause."

Fairfax Media has been told by industry sources of further problems at Ripley, Hemmant and Mt Cotton.

Has Queensland's building industry watchdog let a problem emerge under its watch?

The Queensland Building and Construction Commission rejects suggestions it has done nothing to solve emerging problems in the industry.

QBCC Commissioner Brett Bassett says it has referred the complaints to police and the Board of Professional Engineers.

"The QBCC will take appropriate action once these authorities have concluded their investigations.

"On the invitation of the Gold Coast council and BPEQ, the QBCC attended a site visit last week where the (Gold Coast) retaining wall is located."

It argues it has not received other information from one industry whistleblower.

"The QBCC has asked on multiple occasions for specific information from (the person) in regard to his allegations about issues with other retaining walls. As yet this information has not been forthcoming," Mr Bassett says.

"The QBCC will investigate those matters once the appropriate information has been provided."

Several industry sources with whom Fairfax Media spoke said they had given information to the QBCC, but no investigations had started.

Are bad builders banned?

Under the QBCC operations builders receive between two and 10 demerit points for bad construction.

If a person receives 30 demerit points in three years, they lose their licence for three years.

In 2015-16, 275 were contractors issued with a total of 1730 demerit points.

In the same period 152 individuals and 85 companies lost their licence for financial flaws, and 14 contractors lost their licence immediately for "serious risks" to the community or operating while insolvent.

What whistleblowers say about the problems

Whistleblower 1:

"I showed the Queensland Building and Construction Commission about 12 months ago and said to them, 'Are you going to wait until there is a death?' "

Whistleblower 2:

"The bottom line is that the entire engineering profession is not meeting its responsibilities in policing design and construction in so many areas. It is something you might expect in Bangladesh, but not in modern Australia."

Whistleblower 3:

"No one is policing it. You put in a formal complaint and no one cares less. It doesn't work for builders. It doesn't work for homeowners. It doesn't work for anyone."

Elanora retaining walls near Palm Beach. Photo: Tony Moore

About retaining walls in Queensland

Queensland's Building and Construction Commission says even timber retaining walls should have a 60-year lifespan.

They need to be certified using a Form 15 or Form 16 from the local council.

Problems with certification and collapsed retaining walls have been raised with Brisbane, Redlands, Gold Coast and Ipswich councils over the past 12 to 18 months.

Whistleblowers contacted Fairfax Media in early April after no response from Housing Minister Mick de Brenni, Brisbane and Gold Coast councils or the QBCC.

Retaining walls associated with building works are approved by private certifiers. Retaining walls higher than one metre require approval.

What did Housing Minister Mick de Brenni say?

Mr de Brenni did not directly answer Fairfax Media's questions.

Housing Minister Mick de Brenni with Premier Annastacia Palaszczuk.

A statement from a spokesman said "the Minister's office has repeatedly sought and received advice from the QBCC" and has repeatedly been told that the QBCC has been in contact with one whistleblower".

"The Minister has asked the Commissioner to personally contact one whistleblower to make sure that complaint is fully heard and that 'any action required is immediately taken'."