Australians have been assured the US alliance "matters more today than ever before", as wargames simulating the invasion of the country from the north officially get underway.

Key points: "Talisman Sabre" is the biggest ever joint exercise between US and Australian personnel

"Talisman Sabre" is the biggest ever joint exercise between US and Australian personnel Admiral Harry Harris says the size of the deployment "sends a message to friends, allies, partners and potential adversaries"

Admiral Harry Harris says the size of the deployment "sends a message to friends, allies, partners and potential adversaries" The commander of more than 80,00 US marines in the Pacific urges Australia to join operations against IS in Asia

Exercise "Talisman Sabre" has been formally launched off the coast of Sydney on board the Amphibious American Warship USS Bonhomme Richard.

Over the next month, more than 30,000 US and Australian personnel will conduct their biggest ever joint exercises off the coast of Queensland and the Northern Territory.

"Folks — the Australian-US alliance matters more today than ever before," said Admiral Harry Harris, the head of the US Pacific Command.

"It matters to our two great nations, it matters to the Indo Asia Pacific and it matters to the world."

Talisman Sabre was officially launched off the coast of Sydney. ( ABC News: Taryn Southcombe )

Asked how he thought China would view the exercises, Admiral Harris said the size of the deployment was intended as a signal.

"I'm pleased about the message it sends to our friends, allies, partners and potential adversaries," he told reporters after the official opening ceremony.

Ahead of the Talisman Sabre launch, the commander of more than 80,000 US marines in the Pacific urged Australia to join operations against Islamic State (IS) militants in South-East Asia.

Lieutenant General David Berger is also visiting Australia to check on readiness for the Talisman Sabre military exercises .

He told the ABC the "movement of violent extremist organisations" was a "very real problem" for countries like the Philippines, Malaysia and Indonesia.

Australia's Chief of Joint Operations Vice Admiral David Johnston said the Australian Defence Force had the capability to join the fight on the ground, but stressed it was a "question for government" and no formal request had been made.