By Christopher Knaus and Michael McGowan, originally published on the Guardian on 30 September.

The government is hiding the self-declared conflicts of interest of six departmental and contracted staff who worked in the unit responsible for the $30m Leppington Triangle purchase scandal, including one adviser with links to the business that sold the land at a vastly inflated price.

The explosive audit into the infrastructure department’s 2018 purchase of a dairy farm next to the site of the new Western Sydney airport last week revealed that six staff working in the Western Sydney Unit responsible for the deal had declared conflicts of interest.

One of the staff, a contracted adviser, had declared a conflict with the seller of the land, Leppington Pastoral Company, a firm owned by Tony and Ron Perich, who are billionaire dairy farmers, property developers and Liberal party donors.

That adviser was allowed to continue working on a key project related to the land acquisition – the realignment of a road that would have affected part of the vast property holdings owned by the Leppington Pastoral Company near the proposed airport.

The Guardian has made repeated requests to the department for details of the six staff who had declared conflicts, including for the release of the conflict of interest declaration forms completed by the employees.

But the infrastructure department has refused, saying it would be “inappropriate” for it to preempt an independent investigation and audit now under way. The department has also refused to answer questions about its own investigation, including details about the independent auditor appointed to investigate the deal.

“It would be inappropriate for the department to preempt these processes by commenting further at this time,” a spokesperson said in a statement.

There is no suggestion of wrongdoing by those staff who correctly declared conflicts of interest but the lack of transparency by the government has prompted criticism from both anti-corruption experts and the Labor opposition.

Han Aulby, the executive director of the Centre for Public Integrity, said it should be “compulsory” for government staff to publicly disclose conflicts of interest.

“Taxpayers are funding public officials and contractors to work on their behalf in the public interest – and we have a right to know if other conflicting interests are at play,” Aulby said.

“The refusal of the department to release these details shows again the need for a national integrity commission. Only an independent agency with coercive powers can get to the bottom of these allegations.”

Labor’s shadow infrastructure minister, Catherine King, echoed the need for a national integrity commission to “ensure that dodgy deals like this can never occur again” and accused the Morrison government of downplaying the scandal.

“This scandal is serious, but nobody in the government is taking it seriously,” she said.

“With dodgy deals like this it is hard to have any faith in the Morrison government’s infrastructure program, particularly when the deputy prime minister ticks it off as a bargain and the prime minister only blames ‘process’.

“It is not good enough for the department to hold another investigation into itself. It is integral that there is an independent, transparent investigation to get to the bottom of what happened and why.”

The auditor general last week found shortcomings and ethical failings in Western Sydney Unit’s purchase of the $30m Leppington Triangle, land the government needs for a planned second runway at the Western Sydney airport after 2050.

The audit found taxpayers spent $26.7m too much for the land, which was owned by the Perich family, whose company donated $58,800 to the Liberals in 2018-19.

As part of the audit, the auditor general requested conflict-of-interest declaration forms from 13 officers who were past or present staff of the Western Sydney Unit, two contracted advisers who had provided advice on the Leppington Triangle acquisition and another contracted adviser who had declared a conflict relevant to the land purchase.

It found numerous problems with the interest declarations.

In one case a senior departmental official had not completed a declaration, while a second senior official identified a potential conflict but did not include any further details about it on the declaration form.

In another, a contractor declared a conflict but failed to respond to a request for more information from a probity adviser. The department did not follow up for seven months, by which time the contractor had left.

In a fourth case, a contractor had declared a conflict of interest with the Leppington Pastoral Company. The contractor did not work on the acquisition but was allowed to work on the realignment of a road which would have affected the LPC’s property, a significant concession to the Perich family.

“In the circumstances, the ANAO considers a more prudent course would have been to avoid the conflict by not having the contracted adviser involved in any of the work,” the auditor general found.