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The costings for Scott Morrison's signature super-for-housing scheme were based on scant detail and prepared in an "extremely" short timeframe, according to internal documents which suggest the Coalition's centerpiece policy wasn't finalised until the last minute before the federal election.

The apparent rush meant the estimated cost of setting up the scheme had to be increased by tens of millions of dollars to account for potential blowouts, documents released under freedom of information laws reveal.

Former housing minister Michael Sukkar said the policy was "developed over a long period of time", rejecting suggestions it was pieced together at the 11th hour.

The documents, obtained by The Canberra Times, also reveal the Australian Tax Office planned to set up an "interim system" ahead of the full rollout, which would have involved "limited upfront eligibility checks" on first-home buyers who wanted to dip into their retirement savings to break into the property market.

Unveiled less than a week out from polling day, the former prime minister's super-for-housing plan became the centrepiece of the Coalition's ultimately failed pitch for re-election on May 21.

It would have allowed first-home buyers to withdraw 40 per cent of their superannuation balance, up to a maximum of $50,000, to help purchase a property.

"Super should be harnessed to support the aspiration of many thousands of families who want to buy a home," Mr Morrison said at the time.

Labor lashed the idea as a "desperate act of a dying government", creating a clear point of difference between the two parties in the campaign's final week.

Mr Morrison argued the policy would have only a "marginal impact" on house prices, but refused to release any evidence to support that claim despite intense pressure.

The Canberra Times requested access under freedom of information laws to any analysis or modelling which Treasury had conducted on the scheme.

None of the documents which were retrieved included any modelling on house prices.

But the request did unearth ATO analysis into the estimated cost of setting up and administering the scheme, which was assumed would attract 106,000 applications from first-home buyers each year.

The program was estimated to cost $191 million over the forward estimates.

The majority of the cost was the $132 million required to design and build the internal systems to run the program.

The documents show that $132 million figure included a 50 per cent loading - or "risk uplift" - which the ATO was forced to apply because it had been given an "extremely short period of time" to prepare the costings and due to the "policy specifications not being finalised".

"Without this risk uplift applied, it is highly likely that the full extent of [the] system impacts, and associated costs will not be covered by this assessment," the document stated.

The ATO analysis doesn't include any dates or timestamps, meaning it's unclear how close to the election it was prepared.

However, the documents show senior Treasury officials were still meeting and seeking answers about the plan on May 17 and May 18 - just days before polling day.

The ATO planned to roll out the system to manage the scheme in two phases, starting with a 12-month "interim solution" from July 2023.

Under that system, first-home buyers would lodge their applications over the phone.

It would involve "limited upfront eligibility checks" on applicants, the documents show.

Under the permanent system proposed to be up and running in July 2024, the ATO would conduct up-front checks including on whether applicants had previously owned a home.

The documents also reveal that applicants who withdrew super but did not purchase a property - or repay the funds - within the required timeframe would be slapped with a penalty.

The Canberra Times asked Mr Sukkar to explain why the ATO had to prepare costings for the signature policy based on limited information and in an extremely tight timeframe, and whether the Coalition had left the work to the last minute.

In a statement, the now opposition housing spokesman said the "significant policy" had been developed over a long period of time.

Mr Sukkar again spruiked the benefits of the policy, but wouldn't confirm if it remained part of the Coalition's platform.

"We will announce all our housing policies in good time before the next election," he said.

Dan covers federal politics from Parliament House, with a special focus on climate policy and the NDIS. He has previously reported on ACT politics and urban affairs since joining the Canberra Times in 2018.

Dan covers federal politics from Parliament House, with a special focus on climate policy and the NDIS. He has previously reported on ACT politics and urban affairs since joining the Canberra Times in 2018.

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