At dawn on Saturday, the Carnival Legend and the Pacific Pearl arrived in Sydney Harbour ready to turn around before embarking on New Year's Eve cruises by late afternoon.

Carnival Australia, the nation's largest cruise company, has seven ships offering round trip cruises from Sydney over the New Year's Eve period with a combined capacity of 15,670 guests.

Last year, the company had five liners in service with a capacity of 10,320 but despite the extra capacity the operator said all ships were sailing with full berths.

Royal Caribbean International and Celebrity Cruises, owned by another cruising company, have added another five vessels to the summer line up including a new ship with 1800 berths.

The tug Bullara, capable of pulling 60 tonnes, was waiting off Bradley's Head on Saturday at 6am set to fix its line to the stern of Pacific Pearl. The tug, ready to act as a brake, assisted the liner under the Harbour Bridge with a clearance of just three metres before turning and reversing slowly into a mooring alongside at the White Bay terminal.

Ahead, the massive Carnival Legend equipped with three bow thrusters, and too tall to pass under the Harbour Bridge, made her own way on to a mooring near the Opera House.

Chris Mace, master of the tug owned by operators Svitzer, said when he started as a teenager some 37 years ago the wharves were busy with shipping carrying coal and wheat. Now 60 per cent of the vessels escorted in the harbour were cruise liners, he said.

"Sometimes we have three passenger ships here at once, with one having to stay out on a buoy near Taronga Zoo," he said.

"As other shipping has dropped off, passenger ships are taking over as the main shipping in this port."

Jennifer Vandekreeke, vice president of Carnival Cruise Lines said customers embraced cruising because New Year was about having fun with family and friends.

"Christmas and New Year and early January are the highest demand sailings of the entire year. We have got bookings 20 months out because they are so popular. Carnival Legend, our second ship in Australia, has been full since she got here."

But there's one thing you can't do from a cruise liner and that's watch the Sydney Harbour New Year's Eve fireworks.

"We are certainly asked to leave because we would block the view," Ms Vandekreeke said.

"Basically the ship is a moving 12-storey building."