The port city of Fremantle is enjoying a tourism windfall courtesy of an unscheduled detour by thousands of passengers and crew on board the enormous Queen Mary 2 luxury cruise liner.

Key points: The Queen Mary 2 is carrying 2,000 people on a round-the-world trip

The Queen Mary 2 is carrying 2,000 people on a round-the-world trip The ship changed schedule to avoid cities in Asia over coronavirus fears

The ship changed schedule to avoid cities in Asia over coronavirus fears Fremantle will host the ship two extra nights, delivering an economic boost

The ship was in Colombo, Sri Lanka earlier this month and was due to travel through Singapore, Vietnam and Hong Kong before docking at Fremantle in March.

It stopped in Malaysia last week to refuel and allow 200 passengers to disembark, however no other passengers were allowed to board.

Cruise operator Cunard cancelled the remainder of the South East Asian leg of the round-the-world cruise, citing a desire to "mitigate the risks to our guests and crew" posed by coronavirus travel bans and port restrictions.

Passengers spent two weeks at sea while the itinerary was changed, with the ship ultimately docking at Fremantle on Tuesday, a month early.

It would stay in port for two days before sailing on a revised itinerary through northern and eastern Australia, returning to Fremantle in mid-March.

It meant thousands of passengers and crew would end up spending two more nights than originally planned in the city.

'We're not just sailing in circles'

After two weeks spent at sea, passengers were glad to set foot on land after arriving in Fremantle on Tuesday morning.

"We're grateful we're not just sailing in circles on the ocean," passenger Linda Corcoran, from the United States, said.

Passengers Linda and Bill Corcoran disembarked in Fremantle after two weeks at sea on the Queen Mary 2. ( ABC News )

"To keep us safe they decided not to allow us to go to any of the Asian countries.

"We refuelled in Port Klang [Malaysia] but we did not get off, so we haven't been off the ship for 14 days.

"We're happy to be off but we've been very well cared for."

Her husband Bill Corcoran said they had "nothing to complain about".

"It was a very long trip. We've been on a lot of cruises, this is the first time we've ever been at sea this long at one time," he said.

"Sometimes life throws you a lemon, you just make the best of it."

About 2,000 passengers on board the Queen Mary 2 disembarked at a stop-off in Fremantle. ( ABC News: Frances Bell )

An unusual and costly detour

Maritime historian Chris Frame said Cunard had a reputation for taking a "safety first" approach with its cruises.

"It's quite unusual to see a ship like the Queen Mary 2 make a such a significant diversion from its world cruise itinerary," Mr Frame said.

"These world cruises are planned years in advance so to make a change like that at such short notice is a lot of work for the company and for the officers on board."

Mr Frame said the change would have been costly for Cunard.

"But on the plus side for Australia, it means that they've got all these people on board, over 2,000 passengers [and] 1,000 crew, who are now coming to Fremantle," he said.

The Fremantle Chamber of Commerce hopes the impromptu visit by the Queen Mary 2 will boost spending in the city. ( Supplied )

The Fremantle Chamber of Commerce said having the Queen Mary 2 in the port city twice in less than a month would provide a much-needed boost to the local economy.

"You cannot underestimate the importance of this sort of opportunity," chief executive Danicia Quinlan said.

"The coronavirus has had both negative and positive effects on our local economy here.

"The seafood industry in particular and the live lobster industry is struggling.

"On the upside, [the cruise ship arrival] is a huge positive economic impact on Fremantle and for the businesses of Fremantle that offer that unique experience."

The Fremantle Port Authority said two more cruise ships would make unscheduled stops in Fremantle next month due to similar itinerary changes.