The biggest ship ever built for the Royal Australian Navy has been launched amid pomp and ceremony at the northern Spanish port of Ferrol.

HMAS Canberra and the HMAS Adelaide are amphibious assault ships being built by a Spanish ship-building company, and they are not expected to be in service for three years.

While they are based on an off-the-shelf aircraft carrier design, the ships are billed as a whole lot more.

They do not come cheaply, at about $1.5 billion each, with many billions more being spent on support vessels.

As HMAS Canberra's massive hull slipped into the water in Ferrol following the obligatory champagne christening, a new era in Australia's defence also got underway.

Vice Admiral Russell Crane says while HMAS Canberra looks like an aircraft carrier, it is something else altogether.

He says that instead of fixed-wing aircraft the decks will be filled with helicopters.

"We'll be able to launch six helicopters from the deck at a single period in time, and carry another six in the hangars," he said.

"It has significant capability, but it needs to be seen for what it is.

"It is an amphibious ship that provides assistance across the board in very many different places where you need this sort of capability."

Vice Admiral Crane says it is a proud day for the Navy.

"On the massive open decks below, landing craft will be able to ferry large numbers of fighting vehicles to the shore - from a tank, a truck, to a simple four-wheel drive," he said.

The ship is designed for a force of 1,000 soldiers, fully-equipped and supported.

On hand for its unveiling was former Howard government defence minister Brendan Nelson, who is now Australia's ambassador in Brussels and to NATO.

"I feel like the proud father at the birth of a baby, because we made this decision in 2007 to buy two of these landing helicopters docks and these ships will transform the capability of Australia's defence forces," he said.

"They will be absolutely magnificent for Australia in disaster relief, in crisis intervention. This is just a transformation of Australia's defence capability."

The ships' final fit-out will be completed in Williamstown in Victoria, and their 40 years of service is expected to start in 2014.