Department and agency leaders will be grilled over an AUKUS side pact following speculation the US will not sink the multibillion-dollar security and submarines deal.

A joint committee will scrutinise the Geelong treaty during public hearings at Parliament House in Canberra on Thursday.

The 50-year agreement, signed by Australia and the UK in late July, aims to strengthen bilateral co-operation as part of pillar one of the AUKUS pact.

Under pillar one, the US has promised to sell at least three Virginia-class nuclear-powered submarines to Australia from the early 2030s.

Representatives from Defence, Foreign Affairs and Trade and Attorney-General's departments, as well as the Australian Submarine Agency, will appear before the inquiry.

Retired Royal Australian Navy rear admiral Peter B

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