AUKUS SUBMARINE DEAL

Australia is seeking to replace its ageing Collins class submarines with a fleet of nuclear-powered submarines in the biggest defence purchase in the nation's history.

* Prime Minister Anthony Albanese, UK leader Rishi Sunak and US President Joe Biden are set to announce the details of the deal in San Diego on Tuesday (AEDT).

* Media reports suggest it will be a two-stage rollout, starting with Australia acquiring an initial three US Virginia class submarines in the 2030s, with the option of two more.

* The second stage would involve the building of a UK-designed submarine involving US technology.

* The spending would take Australia's defence bill well above two per cent of GDP.

* It is expected there would be two bases for nuclear-powered submarines, on Australia's east and west coasts.

* Mr Albanese says Australia will retain "absolute sovereignty" over the submarine fleet, as questions arise over the ability to crew the new boats without US help.

* South Australia is expected to play a key role in the building of future submarines, with a number of states involved in the supply chain.

* The deal, which began with the AUKUS pact in 2021, follows the Morrison government's scrapping of an agreement with France on building a new conventionally-powered submarine fleet.