Australia's newest warship, the Adelaide, has been welcomed to its new home at Garden Island in Sydney.

After its fit-out in Melbourne and a program of sea trials, the landing helicopter dock NUSHIP Adelaide berthed at Garden Island, pulling in past its sister ship, HMAS Canberra, which was commissioned last year.

The 230-metre-long air, land and sea amphibious ships, with their ski-jump flight decks, are the largest ever built for the Australian Navy, displacing a massive 27,000 tonnes and costing just more than $3 billion together.

Several Hercules fly over Sydney Harbour as a guided missile frigate heads out to sea. ( ABC News: Deborah Rice )

Once Adelaide is commissioned in December, it will carry about 1,000 soldiers alongside the ship's company and apart from its defence role, it will be deployed for humanitarian duties and disaster relief.

Robyn Croucher, whose son Matthew is a Leading Seaman aboard the ship, said there was a lot of new equipment on board.

"They'll go to sea in December, so I think it will be in and out a lot in the next four years; he's on there for four years now," Mr Croucher said.

"There's a lot of training to go on because the technology on here is different than all the other ones anyway."