A coronavirus outbreak among hundreds of Russian and Ukrainian fishermen flown to New Zealand to bolster its struggling deep-sea fishing industry has prompted that country's largest daily increase in infections in months, authorities said Wednesday.

Over 230 fishermen were flown in from Moscow last week, with 18 of the crew members then testing positive for Covid-19 while in quarantine, New Zealand's director-general of health Ashley Bloomfield said.

The Pacific nation has almost eliminated local transmission of the virus, but regularly records small numbers of new cases in returned travellers. 

The fishing cluster pushed the daily tally of new infections to 25, the highest level since April sparking concern among officials in prime minister Jacinda Ardern's recently reelected government.

New Zealand has recorded over 1,500 cases and 25 deaths in a population of almost five million and has been widely praised for its handling of the pandemic.

Health officials said two new cases had also been linked to a port worker, who may have come into contact with a vessel now docked off Australia's east coast.

Broad travel bans remain in place in New Zealand and those granted exemptions -- including the fishermen, who were classified as essential workers -- are forced to quarantine for 14 days.

Bloomfield said the new cases highlighted the threat of overseas arrivals. 

"We cannot afford to be complacent, we are not being complacent at the border," Bloomfield told media.

The maritime sector has also come under scrutiny in Australia, where dozens of crew members from a cattle ship docked off Australia's west coast tested positive for the virus.

"It is becoming clear that ships arriving with COVID-19 on board is one of the weakest links and the biggest risk to our way of life in Western Australia," Western Australia premier Mark McGowan said Tuesday.

Australia has recorded over 27,400 cases and 905 deaths in a population of 25 million.