The Morrison government has no plan to evacuate citizens quarantined on a cruise ship off the coast of California after fellow passengers tested positive to the novel coronavirus, despite expert warnings it could spread quickly on board.

Four Australians are trapped on board the Grand Princess along with more than 3500 other passengers, after the ship was quarantined near San Francisco when 21 people, including 19 crew members, tested positive for the virus.

Four Australians are quarantined on the Grand Princess cruise ship off the California coast. Credit:AP

Leading virologist Ian Mackay from the University of Queensland said infectious disease experts had learned from the case of Diamond Princess, quarantined off the coast of Japan last month, that cruise ships were "not the ideal environment" in which to quarantine people and that the passengers should be brought to the mainland.

"We know that cutlery and crockery can be fomites [objects that transmit infectious diseases]," Associate Professor Mackay said.