The Ministry of Health is contacting 56 New Zealanders who were on a cruise ship which had four people test postive for Covid-19.
The Ruby Princess, which left New Zealand six days ago, and has had three Australian passengers and one crew member test positive for Covid-19.
The advice will be to be vigilant and contact their GP or Healthline if they display any Covid-19 symptoms.
The ministry is working with Customs and the Ministry for Foreign Affairs and Trade (MFAT) to check which New Zealanders on board have now returned to this country.
The Ruby Princess left Sydney on 8 March, travelled to Fiordland (11 March), Dunedin (12 March), Akaroa (13 March), Wellington (14 March) and Napier (15 March).
The cruise was shortened after visiting Napier, for weather reasons, and the ship returned directly to Sydney and passengers disembarked on 18 March.
Any passengers who returned to New Zealand would be covered by the requirement for all travellers returning to New Zealand to go into self-isolation for 14 days. All returned passengers will be treated as close contacts and followed up with daily health checks, the Ministry of Health said.
The ship carried around 2700 passengers and 1100 crew.
The ministry is working through the itinerary of local visits undertaken by passengers to see if there are other precautionary steps that need to be taken.
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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.
Passengers are not allowed to disembark the ship if they are unwell or showing signs of illness when it docks.
While in Wellington, there were a small number of passengers on board at the time with flu-like symptoms, and five passengers were tested in Wellington for Covid-19 on 14 March. These people never left the ship in Wellington and the results from those tests were negative.
Auckland man who has Covid-19 was on Celebrity Solstice cruise ship
One of the 11 new cases confirmed yesterday - an Auckland man in his 60s - travelled on the Celebrity Solstice around New Zealand.
He Australian cruise ship joined the cruise in Auckland on 10 March and disembarked in Dunedin - before he returned separately to Auckland.
The ship visited Tauranga (11 March), Picton (13 March), Akaroa (14 March), Port Chalmers (15 March) and left from Fiordland on 16 March.
The Ministry of Health's public health staff will be working with their counterparts in Australia to give details of the activities of the passenger while on board the cruise to assist the Australian health services in the follow up they're undertaking there.
The ship arrived in Sydney yesterday and any New Zealanders on board returning to New Zealand will be regarded as close contacts, put in self-isolation and monitored daily.
Health officials say every person who was on that cruise who is now in New Zealand needs to self-isolate now.
As of midnight 14 March all cruise ships were banned from coming to New Zealand until at least 30 June 2020. Ships already in New Zealand waters were permitted to conclude their itinerary.