Mr Gaetjens, who sits in on national cabinet meetings, told a Senate committee that the public briefings delivered by Prime Minister Scott Morrison about the deliberations of the nation's federal, state and territory leaders were open and comprehensive.

"The statements by the Prime Minister and the outcomes of the decisions that have been taken have been the most transparent I have ever seen," the top public servant said.

Mr Gaetjens was responding to questions put by Labor, Liberal and crossbench senators including Katy Gallagher and Rex Patrick regarding how the national cabinet, which is co-ordinating the national response to the COVID-19 crisis, operates.

The national cabinet, which comprises all federal, state and territory leaders as well as Chief Medical Officer Brendan Murphy and Mr Gaetjens, has been meeting at least weekly since the health emergency escalated in early March.

At its meeting last Friday it agreed on a three-step plan to ease restrictions on personal gatherings, businesses, education and travel developed by the nation's top health officials.

Senators grilled Mr Gaetjens and Professor Murphy on who advises national cabinet, how it makes decisions and whether states and territories are required to abide by its determinations.

National cabinet's status has come into increasing focus in recent weeks as states and territories have adopted significantly different approaches to re-opening schools and relaxing physical distancing and business restrictions.

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The government has insisted there is broad consensus on the policy response and Professor Murphy told the hearing there had not been any "material" disagreement within national cabinet on the health advice.

But this was challenged by senators given the range of state and territory actions and apparent backflips like Mr Morrison's change of advice regarding hairdressers.

On March 24 the Prime Minister advised that hairdressers would remain open but appointments would be limited to no more than 30 minutes. He later removed the time limit after feedback that it was impractical, Professor Murphy said.

Mr Gaetjens said there was no inconsistency between the national cabinet agreeing on health principles but states and territories implementing changes to restrictions to suit their individual circumstances.

The issue leapt to prominence last week when federal Education Minister Dan Tehan lashed Victorian Premier Daniel Andrews for deciding to keep schools closed despite Commonwealth urgings to open them.

Mr Gaetjens said the national cabinet was formed as a sub-committee of federal cabinet and was primarily advised by the Australian Health Protection Principal Committee, though it had also received briefings from Treasury, the Reserve Bank of Australia and the Mental Health Commission.

He said the decisions of national cabinet were ratified by the federal cabinet.