Auburn council voted to sell its colleague Salim Mehajer prime council land despite a month earlier commissioning its own valuation that found the land was worth $5 million more than the sale price.

"It's scandalous," said Labor councillor George Campbell.

One month ago the council, led by Cr Mehajer's close ally Ned Attie, voted to sell to its colleague a car park in John Street, Lidcombe.

Cr Mehajer plans to turn the block into a 96-unit residential complex with additional retail space.

Cr Attie's motion had the council sell the car park at a value of $8.5 million, according to a price recommended to council by McGees valuers.

But Fairfax Media can reveal that the council commissioned a second valuation of the property by property giants CBRE that put its value at a significantly higher price.

The terms of that valuation are confidential, but sources have told Fairfax Media that it exceeds $13 million.

Soon after the sale was passed by the council, Fairfax Media contacted a number of independent property valuers, who estimated the value of a site of that size exceeded $13 million. This is the first time it has been revealed that the council itself had already received a complete valuation showing the property could have fetched a greater sale price.

Opposition councillors have said they failed to realise a second valuation had been obtained and that it was "buried" in documents provided to councillors.

None of the four councillors who voted for the sale responded to detailed written questions from Fairfax Media asking why they voted to sell public land to a colleague at an apparently steep discount.

Councillors were handed on the night of a meeting the month before the debate on the sale a document of about 50 pages. Only about 10 pertained to the higher valuation.

Two councillors have told Fairfax Media they mistook the second valuation for an attachment.

"The information was buried at the back of what seemed to be one [quote]," Cr Campbell said. "It was disappointing that the information did not draw attention to the inclusion of two [quotes].

"The decision to sell was led by Cr Attie."

The council was deadlocked on the vote 4-4, leaving Cr Attie to use his casting vote as chair of the meeting to get the motion across the line.

A council spokesman disputed that the inclusion of a second valuation was unclear and said it was explicitly mentioned in documents.

Independent councillor Irene Simms said she would push to overturn the motion when the council reconvened in mid-February.

Unusually, council staff made no recommendations on which option councillors should choose as is custom when presenting briefs to councillors.

Nor did they draw the council's attention to the second valuation when  Cr Campbell argued in favour of seeking a second opinion, saying he had been advised by a real estate agent the property was worth well over the proposed sale price.

Mayor Le Lam, who claimed to have only recently become aware that her brother-in-law Minh Hua and Cr Mehajer were in business, recused herself from the vote.

Crs Attie, Mehajer and Le form a bloc of votes on council, self-styled as the "super six" that controls a majority of votes.

Cr Mehajer also did not take part in the vote. He said he was unaware of the second valuation.

"As a councillor I shouldn't be looking into my own items," he said.

The council had previously voted to sell the car park to Cr Mehajer for $6.5 million, but that decision was overturned following a public outcry.

For the deal to go ahead it will now require a new development application to be submitted to and win approval from a Joint Regional Planning Panel, a state government panel that considers larger planning proposals.

Though independent from council, two of its five members are Auburn councillors, including Crs Attie and Hicham Zraika.