Detectives say they will reopen discussions, but have yet to commit to searching a block of land the Phillips siblings say might hold critical evidence, or potentially the body of their sister.

Bob Dallow, a retired detective inspector who devoted decades to investigating Sharron's suspected murder, agrees that the alibi is questionable.

And the detective's wife, Kay Dallow, says she was given a chilling admission by a heavily sedated and ill mother of Sharron: "Bobby killed Sharron and put her in a box. And I want him to kill me and put me in a box too," she was alleged to say at her son "Charlie's" wedding in 1991.

Sharron's body has never been found and no one charged, despite hundreds of leads since 1986.

A block of bushland right next to the family home in Riverview, next to St Peter Claver College, must now be searched for the first time, the family and the retired detective say.

On Thursday, the head of Queensland's Cold Case Squad, Acting Inspector Glenn Terry, agreed to send an investigator to meet the Phillips' siblings after their questions were explained to him.

"We'd like them to check that bush beside my parents' house," Donna Anderson (nee Phillips) says.

"It's never been searched." Darren Phillips, now 49, says.

"Not, ever," says "Charlie", now 51, calmly describing the investigation 30 years ago as "chaotic".

Inspector Terry said: "Anything that we can take from the family, or any other witnesses that may bring the matter to a successful resolution, is certainly of interest to us."

Will the Riverview block be searched?

Inspector Terry said a decision on searching the Riverview block depended on the information investigators received.

"It is very costly, so we have to weigh up the information received and look at whether it is going to take us anywhere and do an evaluation from that point," he said.

Dallow said that last Friday he asked Assistant Commissioner Brett Pointing – whom he knew when they were both junior detectives – to have the Riverview bushland searched.

Family doubts

Sharron Phillips' brothers and her older sister doubt their father's claim that he was collecting "one of their trucks" from the New South Wales town of Gilgandra the night Sharron disappeared.

Fairfax Media met with Sharron's older sister Donna and brothers "Charlie" (christened Robert) and Darren on Tuesday night in an outer Ipswich suburb. Donna was 23 at the time of Sharron's disappearance; Charlie was 22 and Darren 19.

They are the oldest of the nine Phillips children – there are five older children and four younger siblings.

They called their sister "Big Bird". It was the family's nickname for the young woman who was approaching her 21st birthday.

They say she dressed well – "she knew how to make herself look nice"  – was delighted to be living "out of home" in her own unit at Archerfield and "always, always" spoke her mind.

"Sharron didn't wear rubbish. She knew how to dress nicely.  She knew how to dress herself up," Darren said.

Thirty years on they said they had never been interviewed by police about Sharron's disappearance. They wanted to speak with police.

The trio were not accusing their father, but said they had very good questions about his alibi that needed answers.

"I had tea with Sharron (at her Archerfield flat). I went out and seen (sic) Sharron on the Wednesday night before she went missing on the Thursday night," Darren said.

"And Sharron said to me that the old man had the shits with both of us, because we'd moved out," he said.

"And he didn't know if he could afford the house with us moving out, because he's lost our money type of thing."

Donna said she first raised questions with police 10 years ago, but had struggled to get a police officer to listen, providing to Fairfax Media a list of officers and stations she had called to no avail.

Their father's alibi

In the 1980s and early 1990s the Sharron Phillips case had a massive media profile, similar to Daniel Morcombe's disappearance in 2003 or Allison Baden-Clay in 2012.

Sharron Phillips, 20, went missing on May 8, 1986.

Sharron's yellow-coloured Datsun Bluebird ran out of petrol about 11pm near the Wacol Migrant Centre on Ipswich Road and she called her boyfriend for help.

Late family patriarch Bob Phillips had told a reporter he was in NSW picking up "one of his trucks" the night Sharron died.

"I was picking one of our trucks up at Gilgandra (700 kilometres south of Brisbane, near Dubbo). I was in Gilgandra, with Dawn," he told reporter Matthew Condon in 2006.

"We got back about four, five o'clock on the Friday morning. I crashed and went to bed then the story came up and I started ringing everybody to find out what's going on."

What Bob Phillips said

Bob Phillips died in August 2015. He said he had nothing to do with Sharron's disappearance.

"They never actually interviewed me at all," he told Mr Condon in 2006.

"They interviewed Dawn. They had a yarn to us on Saturday morning, but it's only natural they looked at the parents. I was pretty well known here and in Inala so I had nothing to hide. Not a bloody thing."

What the family said

Donna Anderson told Fairfax Media it was just not true that her father "owned trucks" at the time Sharron went missing.

"Because when I began to think about it, about 10 years ago, that he had an alibi because he was picking up a truck, I couldn't believe it, because it's just not true," she said.

Her brothers agreed. Charlie Phillips was living on the ground floor of their Riverview home in May 1986.

He said their father sold their old Dodge truck years before Sharron's disappearance.

"He got rid of that truck before any of that, long before any of that happened, and he went back on a pension," he said.

"The point is that there was no truck at the time. There was no 'trucks', he didn't work for anyone. He didn't 'do' anything."

Donna said she and her brothers and sisters believed that was why police were reluctant to talk to them.

"Because these basic things were not done and they don't really want to solve the case."

Charlie said he remembered his father coming home that night in May 1986, but he can not remember what time.

What the retired detective believes

Bob Dallow, who now runs a bookshop at Ashgrove, twice confronted Bob Phillips – finally in hospital before his death – about his role in Sharron's disappearance.

Dallow now seriously questions Bob Phillips' alibi.

"The other thing is this: Who brought his car back? Because Dawn couldn't drive," he said.

Mr Dallow's wife Kay told Fairfax Media a fragile and heavily medicated Dawn Phillips told her at Charlie's wedding in September 1991 that Bob Phillips had killed his daughter.

"She was very upset and the wedding was due to start and I said to Bobby (Phillips), 'I'll stay and sit with Dawn, while you go and support your son at the wedding'," Ms Dallow said.

"Dawn said to me, 'Kay, Kay. Bobby killed Sharron and put her in a box. And I want him to kill me and put me in a box too."

Mr Dallow said the comments were very hard to investigate because Dawn spoke "rubbish" by the end of her life. Bob Phillips always dismissed her comments as confused by her medication.

Bob Phillips died on August 23, 2015, aged 75. Dawn Phillips died in 2010, after decades of taking lithium for depression.

After Bob's funeral last year the older brothers and sisters began talking among themselves for the first time in 27 years about Sharron's disappearance.

Questioning their father

They have all agonised about suspecting their dead father.

"I have felt so bad for thinking it may have been Dad," Donna said.

"And then I get angry because I think if the police had asked these questions 30 years ago, we wouldn't be sitting here now and I would not have been tormenting myself for the past 10 years."

Charlie Phillips asked: "How can it be wrong just to have the case reopened and for them to have another look at this bloke? Can you be wrong?"

Darren Phillips said many questions – prompted by an opened bag of lingerie at Sharron's Archerfield unit – also worried him.

"I was 19. I'm 49," he said.

"This has been going on longer than my actual life I had beforehand. So I've spent a major chunk of my life defending this man.

"And then I find out at his funeral (2015), I find out the number one detective accuses him.

"And you start asking questions."

The decades-long investigation sadly splintered the nine children until Bob Phillips' funeral.

They declined to be photographed, saying they did not want their children and grandchildren impacted by investigations.

The family said they were not interested in, or prompted by, any reward money. 

Further information

There is still a reward for $250,000 for information about Sharron Phillip's disappearance. An indemnity for a person "not being the person who committed the crime" is available.

People can offer information to Crime Stoppers on 1800 333 000, or the Homicide Squad on 3364 4150.