Independent Senator Rex Patrick has claimed a "magnificent victory" for transparency and urged the Prime Minister to change the way he does business after the Administrative Appeals Tribunal found national cabinet is not a "cabinet".
The landmark ruling on Thursday may be subject to appeal, but it essentially means documents from the currently weekly gathering of commonwealth, state and territory leaders are not privy to cabinet-in-confidence protections and are now subject to freedom of information laws.
Senator Patrick brought the case with pro bono legal assistance in a bid to unlock the deliberations and decision of the grouping of leaders as they guide Australia's response to the COVID-19 pandemic.
"This is a magnificent victory for transparency for accountability and for making sure that the Prime Minister operates within the rule of law," the South Australian senator said.
"It's a scathing decision. It basically states that the Prime Minister, or the government did not properly establish the national cabinet.
"It doesn't have the membership of a cabinet. It doesn't have the collective responsibility of a cabinet. It doesn't have responsibility to one Parliament as a cabinet should have."
The Morrison government is reviewing the decision. There are FOI exemptions of commonwealth and state relations which could still keep deliberations secret.
Senator Patrick says Australians are entitled to see how the pandemic is being handled and believes the ruling will have huge implications for the Morrison government.
"They are now going to have to be much, much more transparent than they previously had. And that is a good thing," he said.
"The entire purpose of me mounting this challenge was to open the vault and to bring the Prime Minister to operate within the laws of this country. The Prime Minister arrogantly laid out a secrecy blanket over a significant body of government and he did so unlawfully. And I suspect deliberately, and that's not acceptable.
"The Prime Minister needs to change the way he does business. He can't operate in this arrogant manner."
A spokesperson for the Morrison government told The Canberra Times the decision will not stop the grouping's important work.
"National cabinet has worked closely to save lives and livelihoods throughout the pandemic and it has been a critical reform that's guided Australia's response," the spokesperson said in a statement.
"The Prime Minister publicly discloses the discussions and decisions of national cabinet after every meeting.
"The government will review today's decision, but will not let it impact the important work national cabinet is doing."
The gathering of commonwealth, state and territory leaders is due to meet on Friday.
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