Former Prime Minister Scott Morrison apologies for secret ministerial appointment scandal, insists it was done in good faith

But the under-fire former prime minister made no attempt to explain why he did not publicly disclose the appointments, which he claimed were "there as a 'break glass in case of emergency' safeguard".

In a thousand-word open letter on Facebook on Tuesday, Mr Morrison insisted the COVID-19 pandemic necessitated "extraordinary measures to respond", but claimed he did not remember being given authority over the treasury and home affairs portfolios until recently.

Mr Morrison claimed he only used his extraordinary powers once - to scuttle a controversial NSW gas project - and that he never attempted to interfere with ministers' "conduct in their portfolio".

"I have endeavoured to set out the context and reasoning for the decisions I took as Prime Minister in a highly unusual time," he wrote.

"I did so in good faith, seeking to exercise my responsibilities as Prime Minister which exceeded those of any other member of the Government, or Parliament.

"For any offence to my colleagues I apologise."

Prime Minister Anthony Albanese earlier revealed Mr Morrison secretly assumed control over five portfolios - finance, health, resources and energy, treasury, and home affairs - and did not relinquish authority before the May election.

The latter two were revealed by Mr Albanese on Tuesday, and Mr Morrison claimed he did not remember being given power over them.

"In hindsight these arrangements were unnecessary and until seeking advice from the Department of Prime Minister and Cabinet today, I had not recollected these arrangements having been put in place. There was a lot going on at the time," he wrote.

Then-treasurer Josh Frydenberg was reportedly unaware Mr Morrison had assumed control of his portfolio. Only former health minister Greg Hunt, and former resources minister Keith Pitt, have admitted knowing about the arrangements at some stage.

Mr Morrison said there was "no guidebook" on how to handle a crisis like COVID-19, and claimed the measures were necessary to ensure he could exercise a minister's function if they were struck down by the virus.

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He did not mention the pre-existing ability for ministers to serve in an acting capacity while a permanent minister was out of action. Under such circumstances, parliament is informed.

"In the event that I would have to use such powers I would have done so disclosing the authority by which I was making such decisions," he wrote.

"The authority was pre approved to ensure there would be no delay in being able to make decisions or take actions should the need arise."

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