New Zealand Customs is undertaking an investigation into the Ruby Princess cruise ship to establish whether any offences have been committed, Prime Minister Jacinda Ardern has revealed.

Ardern made the announcement alongside Education Minister Chris Hipkins at today's post-Cabinet press conference.

The Ruby Princess 13-day return trip to New Zealand included stopped in Dunedin, Akaroa, Wellington and Tauranga.

It arrived in Dunedin on the morning of March 12 and departed the same day, at 6pm.

The Ruby Princess docked at Port Chalmers earlier this year. Photo: Otago Daily Times

After arriving in Sydney passengers were allowed off when many were feeling ill.

Covid-19 tore through the ship and sparked cases in Australia and New Zealand. Sixteen cases in Hawke's Bay have been linked back to the ship which docked in Napier on 14 March.

Since disembarking in Sydney, 18 passengers from the ship have died and hundreds of confirmed cases have been linked to the vessel.