Australia's former prime minister Scott Morrison said on Wednesday he decided to be secretly sworn in to five key ministries during the pandemic because he felt the responsibility for the nation was his alone.

Morrison, who stepped down as leader of the Liberal Party after losing a general election in May, addressed a news conference to answer a barrage of criticism from the Labor government and his own party over the unprecedented move.

Prime Minister Anthony Albanese said Morrison had attacked the Westminster system of government by secretly appointing himself to five portfolios including home affairs, treasury, health, finance and resources between 2020 and 2021.

Three of the ministers were unaware Morrison shared power over their ministries until the revelations this week, they said.

"I was steering the ship in the middle of the tempest," Morrison told reporters, recalling how the pandemic hit Australia in 2020.

"As prime minister only I could really understand the weight of responsibility that was on my shoulders and on no one else," he added.

Morrison said he didn't "take over" the ministries, and said no ministers were interfered with except on one occasion, where he rejected a resources project.

He said he did not inform the ministers because he would only use the powers in an emergency.

Treasurer Jim Chalmers said earlier on Wednesday Morrison's behaviour was "dictatorial" and whether he remained in parliament was a test of Liberal Opposition leader Peter Dutton's leadership.

Morrison rejected this, and said the emergency powers were put in place by a democratically elected prime minister and according to law.

Morrison's secret accumulation of ministerial roles began with the health and finance ministries in 2020, which he has said was because of concern these ministers could be struck down with COVID-19 while wielding emergency powers that could not be checked by parliament.

In May 2021, Morrison also became home affairs minister and treasurer.

Morrison said this was partly due to the national budget being delivered in May and ongoing negotiations with Britain and the United States over the AUKUS security agreement for nuclear submarines.

The resources minister role he took on in 2021 was different to the others, and the only role in which he triggered ministerial power - to stop approval for a gas exploration project off the coast of Australia which was opposed by local communities. The decision is being challenged in court.

"This is fundamentally a trashing of our democratic system," Albanese said on Wednesday, after listening to Morrison's press conference.

The solicitor general will provide advice on the matter to Albanese on Monday.