The auditor general directly warned the attorney general, Christian Porter, that the government’s slow handling of an unprecedented attempt to suppress his work could “undermine the integrity of my office and me”.

Personal correspondence between Grant Hehir, the auditor general, and Porter shows frustration at the months it was taking the Coalition to decide on a bid by a multinational arms manufacturer to suppress a critical audit report.

Guardian Australia revealed last month that the report found Australia could have paid half the amount for its new $1.3bn combat vehicle fleet if it went offshore.

In January the manufacturer, Thales, asked Porter to use – for the first time – powers to suppress Hehir’s audit arguing its release could threaten Thales’ commercial interests.

It took Porter six months to make a decision. The delay left the auditor general in a difficult position. He is legally obliged to give his reports to parliament as soon as possible, and defence continued procuring the Hawkei combat vehicles as the months dragged on.

Wealthy Queensland family lobbied for inland rail line to run past their airport Read more

In May, after waiting four months for a decision, Hehir wrote to Porter and warned him the delay risked setting an “undesirable precedent”. Hehir said it could be viewed as a means for the government to “interfere (or be perceived to interfere) with my statutory responsibility to the parliament by delaying the tabling of an audit report”.

“I believe that it is in neither of our interests for the continuation of an open-ended process for you to consider the issuing of a [suppression] certificate,” he wrote in correspondence given to the joint committee of public accounts and audit.

“For me it has the potential to undermine the integrity of my office and me, in my role as an independent officer of the parliament, in the eyes of the parliament.”

Hehir noted there were long periods between Porter receiving the complaint from Thales and requests for relevant information from his cabinet colleagues and the defence department.

But Porter told Guardian Australia that there was no unreasonable delay. He said the powers to suppress parts of the report, contained in section 37 of the Auditor General Act, were an important check and balance “designed to ensure national security and other interests of the commonwealth are maintained”.

“The process allows for a certificate to be sought in this case by the company in question and further allows for other responsible ministers to make submissions if they consider there are national security grounds for redactions,” he said. “That is exactly what happened here and the time taken to receive and consider those submissions was not unreasonable and, in fact, in total accordance with the process set out by parliament in the auditor general’s own act.”

The correspondence also reveals that Hehir had tried to give parliament an interim redacted report on the Hawkei vehicles, but met resistance from Porter. Hehir told Porter that this had both “surprised and concerned” him.

“I am both surprised and concerned that in your correspondence of 7 May 2018 you suggest that tabling an interim redacted report ‘would pre-empt and attempt to prejudge my (ie your) decision in this important matter, and amount to an abuse of process that would impact all interested parties, including the standing of the ANAO [Australian National Audit Office]’,” Hehir told Porter.

“It was my considered opinion that tabling an interim redacted report… would fulfil my statutory responsibility to the parliament without compromising the interests of other parties.”

The correspondence from Hehir to Porter has been handed to an inquiry into the controversy, which is being conducted by the joint committee of public accounts and audit.

Hehir has previously told the inquiry that requests to suppress his reports should be dealt with within specified timeframes.

The committee’s deputy chair, Labor MP Julian Hill, said that proposal “seemed reasonable”. He said Labor would seriously consider changes to strengthen the laws governing the auditor general and his office.

“The release of the correspondence backs up the auditor general’s suggestion that the parliament consider introducing a time limit for section 37 certificates so the attorney general can’t manipulate the process,” he said.

“Given what has happened, which is unprecedented, this seems to be a reasonable proposal.”

Proponents of the Hawkei argue Australia is better served by having combat vehicles built in Australia, both for the benefits to local industry and the ability to adapt the vehicle to Australia’s unique and changing needs.

Thales said these benefits were ignored in the audit report. It also said the auditor general improperly compared the Hawkei to the rival joint light tactical vehicle (JLTV) in the US. It said the two vehicles were not comparable.

Thales pointed to the success of the Bushmaster – the well-respected light protected vehicle used by Australian forces in various theatres – as evidence of the benefits of local production.

• This reporting is supported by the Susan McKinnon Foundation through the Guardian Civic Journalism Trust