Revelations that Australia's ex-prime minister secretly appointed himself to several ministerial posts during the pandemic sparked a political firestorm on Monday, with his successor promising a rapid investigation.

Prime Minister Anthony Albanese accused Scott Morrison of "tin-pot activity" after it emerged the former leader had made himself minister of health, finance and resources, among other positions, without informing colleagues, parliament or voters.

Describing Morrison's actions as "extraordinary and unprecedented," Albanese said on Monday he had sought legal advice from the ­solicitor-general. "Scott Morrison was running a shadow government," Albanese said. 

In some cases, Morrison made himself a co-minister without telling the cabinet members he had already appointed to those positions.

The scandal has shone a light on the opaque nature of decision-making inside Australia's ­government - and raised questions about ­whether more stringent democratic safeguards are needed.

It is still not clear how many posts Morrison held, but local media reported that he took on the resources portfolio and used his power to axe a significant gas project off Sydney's coast.

Morrison's conservative coalition lost power in May elections, ending nearly a decade of center-right rule in the country.

In Australia, elected politicians are selected by the prime minister before being sworn in by the governor-general in a formal ceremony that is usually publicly recorded.

Constitutional law expert Anne Twomey described the allegations as "bizarre" and said it raised possible legal challenges to some of the former government's decisions.

"The secrecy involved in this is just simply bizarre. I mean, you know, you just wonder what's wrong with these people, if they have to do everything in secret," she said.