Malcolm Turnbull has expressed confidence in his ministers, amid concerns on his frontbench about damage from a national security leak.
Former prime minister Tony Abbott has denied being the source of a leak to The Australian of a document showing new submarines were initially planned to come into service in the mid-2020s.
Mr Abbott said he was disappointed and "flabbergasted" at the Turnbull government's plans to have the new submarines in service by the early 2030s.
"If I've got something to say, I say it (publicly)," he said.
Treasurer Scott Morrison said the government was taking the leak seriously by holding an inquiry.
"Playing politics with national security, I think, is a very, very dangerous and disappointing thing to have occurred," Mr Morrison said.
He said he was prepared to take Mr Abbott at his word.
Opposition Leader Bill Shorten asked the prime minister in parliament on Thursday if he could "explicitly express confidence that this leak has not come from any current or former member of the national security committee of cabinet".
"Of course I have confidence in all of my ministers," Mr Turnbull said.
"The leader of the opposition should consider whether he wants to continue to make mischief out of what is a very serious matter."
Cabinet minister Mitch Fifield said it undermined cabinet confidentiality, which was important when dealing with issues of national security.
Deputy Prime Minister Barnaby Joyce told 2GB radio that unity was important especially heading into an election later in the year.
"I think we've got to all get to get together as one," Mr Joyce said.
"We've got to go to the election unified ... quite obviously honeymoons don't go on forever."
Foreign Minister Julie Bishop gave a blunt warning to leakers.
"It's a criminal offence," she said.
Former defence minister David Johnston said the timeframe for the submarines was not surprising.
"It's a 30- to 40-year program, a four-year delay is nothing. There's no delay," he told Sky News.
Mr Shorten told parliament Mr Turnbull was now fighting off an "insurgent former prime minister" who was critical of many aspects of government policy as he defended his legacy.
Liberal senator Zed Seselja defended Mr Abbott's recent comments on the economy, tax, national security and foreign policy.
"Former prime ministers are entitled to speak - no one should be gagging them," Senator Seselja told Sky News.
"But we all need to consider, whenever we speak, whether it's constructive or not."
Asked by Labor frontbencher Kate Ellis about Mr Abbott's "full frontal assault", Mr Turnbull took aim at Labor's negative gearing policy which dominated the final day of a fortnight of parliament.
"What this constitutes is a full-frontal assault on business in Australia," he said.
