The Ministry of Health has announced 56 New Zealanders were on board an Australian cruise ship where four people tested positive for Covid-19.

The Ruby Princess left New Zealand five days ago and docked in Sydney on Wednesday.

Three Australian passengers and one crew member were confirmed to have the potentially deadly virus.

The Ministry of Health says they are in the process of contacting all New Zealanders who were on board the ship. The advice will be to be vigilant and contact their GP or Healthline if they display any COVID-19 symptoms.

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Focus: How to stop the spread of coronavirus. Video / AP / Mark Mitchell

The cruise ship departed Sydney on March 8. Any passengers on the cruise, who had since returned to New Zealand would be covered by the requirement since March 15 for all travellers returning to New Zealand to go into self-isolation for 14 days.

The Ministry says they are working with Customs and the Ministry for Foreign Affairs and Trade to check whether Kiwis on board have now returned to New Zealand.

As an additional step, all those returned passengers will now be treated as close contacts and followed up with daily health checks, the MoH spokesman said.

The ship carried about 2700 passengers and 1100 crew.

The ship travelled to Fiordland on March 11, Dunedin on March 11, Akaroa March 13, Wellington March 14 and Napier March 15.

By the time the cruise reached Napier the trip was shortened, for weather reasons, and returned directly to Sydney and passengers disembarked on Wednesday.

It's understood crew were on board the Ruby Princess before it docked in Sydney and passengers were allowed to disembark.

The Ministry of Health is working through the itinerary of local visits undertaken by passengers to see if there are other precautionary steps that need to be taken, the spokesman said.

Passengers disembark from the Ruby Princess at Overseas Passenger Terminal on February 08 in Sydney, Australia. Photo / Getty Images
Passengers disembark from the Ruby Princess at Overseas Passenger Terminal on February 08 in Sydney, Australia. Photo / Getty Images

Doctors tested 13 unwell patients and three found to have the virus.

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He said ministry staff will also be working with their counterparts in Australia to determine – from the four cases with COVID-19 – any other possible close contacts in New Zealand that will be contacted, put into self-isolation and tested if showing symptoms.

"Two of the three positive results were people who were passengers on board the ship," NSW Health Minister Brad Hazzard told reporters on Friday.

"One of those passengers was not at all well, and was taken off the ship and has been taken to a hospital here in Sydney, and is being cared for. That particular passenger, now patient, is not particularly well."

On board there were some people on board the cruise ship who presented with flu-like symptoms," Hazzard said.

The ship has already left Sydney and is currently at sea, off the New South Wales coast, somewhere between Sydney and Wollongong.

Hazzard said it was very important for those who had left the ship to self-isolate in their homes for 14 days.

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"If you start to show any of the symptoms, whether it a fever, a cough, or any of the other well-known symptoms that can associate with - like flu, obviously - we want you to make sure you report that in, and take due medical advice.

"It is actually serious because if we have nearly 2700 people that were passengers on that ship, we want to know every single one of them is in quarantine."

He said contacting nearly 2700 people was "a bit of a journey" and some had not responded.

"I'm asking the media to get that message out to the community. And to the community: If you know somebody who came in yesterday from the Ruby Princess, do our community a very big favour and have a chat and make sure that they are given this clear message.

"Put yourself in a self-isolation. Follow the rules, and if you're feeling ill, any of the symptoms that might be related to Covid-19, then make sure you contact a medical practitioner, either your local GP or the local ED in the hospital, and then take advice on what you should be doing."

The number of Covid-19 cases in NSW has increased by a "substantial" 75 people to reach more than 380.

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Hazzard said there had been an extra 75 cases in the 24 hours to 11am taking the total to 382.

"It's obviously quite a substantial increase and, again, it's indicative of the growing issue that faces the entire world," the minister told reporters in Sydney.