Unprecedented growth in Brisbane's unit stock has seen the number triple since the turn of the century, with a 2015 peak in unit construction in the inner city.

An inner-Brisbane real estate agent, who spoke on the condition of anonymity, said self-interest dictated that most agents would not publicly discuss the trouble facing the industry.

"The unit market is flooded with oversupply," the source said.

"The biggest problem is buildings with off-the-plan sales, which were purchased with 10 per cent deposits.

"These were sold at the peak of the cycle. You had tiny one-bedroom apartments selling for $450,000."

Some of those were now being valued at as little as $320,000, the source said.

The Hudson in Albion was considered a canary in the coal mine, the source said.

There, apartments were advertised with discounts of up to 39 per cent, prompting a stoush between the developer and a marketer.

That led to fears, the source said, that many buyers would simply cut their losses, rather than incur the long-term debt associated with overvalued properties.

Lenders would also be unwilling to stump up the cash at settlement, due to their valuations falling well short of the purchase price.

"They either have to bail on the 10 per cent or borrow elsewhere," the agent said.

Developers were also turning to mezzanine finance, the source said, as a means to get projects off the ground.

Urbis associate director Paul Riga, a professional analyst of the Brisbane market, said there had been little evidence of settlement issues in Brisbane so far.

But Mr Riga said it was clear the market was in the midst of a major slowdown.

"We saw a record number of new apartments come into inner Brisbane in 2016 and that was a market that was, from what we've been able to gauge, relatively successful in the sense that there weren't any major issues from a settlements perspective," he said.

"The majority of projects have settled around the purchase price."

Mr Riga said the market was correcting after a period of unprecedented growth.

"It was an aggressive cycle," he said.

"It came around quite quickly, in the sense that it was driven by the lack of supply that happened after the GFC.

"It was quite a steep and aggressive peak that we saw and, equally so, it's been a relatively steep and aggressive slow-down.

"I haven't seen something of this speed come through, but the scale of it indicates that it's a typical way a market moves, with supply and demand coming on."

Mr Riga said new stock had become harder to offload as a result.

"The number of off-the-plan sales we've seen has certainly come down quarter on quarter and a lot of that is due to the fact that we're in a market that's firmly in the delivery phase," he said.

"There's a fair bit of product currently under construction and that's set to come on this year and early next year, so a lot of investors are playing a bit of a wait-and-see game, to see how the market performs with this new product coming on."

Real Estate Institute of Queensland spokeswoman Felicity Moore said it was hard to predict the scale of the slide, or how long it would last, but those looking to get into the market could take advantage of the situation.

"What we know is that the projected number of apartments that were meant to come online are not, because developers have either pulled the pin, on-sold the block or done a whole range of things that means supply is slowing down," she said.

"... For anyone who wants to get into those areas close to the city, they should be using some of that negotiating power to get a good bargain.

"That's what those agents should be saying. They're the ones at the coalface of this and I don't know how jittery they are, but our view, from what we see on a daily basis, is this is an expected softening and it's not even going to be that long-term."

A copy of the flyer advertising apartments in The Hudson development in Brisbane's Albion. Photo: AFR

According to REIQ figures, 2300 new apartments came on to the Brisbane market in the first quarter of 2015.

In the first quarter of 2017, that number had reduced to 260.

"Our view is that the market is cyclical, that this was always going to reach this point - people have been talking about it for well over two years - but we're not sure how long (the downturn will last)," Ms Moore said.

"We don't think very long.

"We think that demand is growing steadily, population growth and the net migration rate remains in positive territory and as we start to see some projects ... coming into the city centre, there's a consistent and steady reason to move into the inner market and that's going to boost the demand side of the equation.

"So while supply has briefly outweighed demand, that demand will resurge in the short-to-medium term."

Despite all the talk of doom and gloom, Mr Riga said the owner-occupier market had remained stable throughout the slide.

"From what we're gauging from our developer clients and developers we deal with, there seems to be a bit more interest from the local perspective, so local buyers are looking," he said.

"Now there's supply coming through, they're starting to look at their options in those areas."

Ms Moore said she expected the inner-city market to improve significantly over the next decade.

"Brisbane is on the cusp of an exciting period right now," she said, citing Queen's Wharf, Howard Smith Wharves and the proposed Brisbane Live precinct as potential drawcards for buyers.

"Over the long term, for people who have bought, if they see through the asset growth they'll be in positive territory in due course."