Air New Zealand is keeping secret the number of its staff infected with Covid-19 amid allegations it is not doing enough to keep its workers safe.

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The airline's crews who fly internationally continue to be exempt from the strict 14-day quarantine rules for people returning to New Zealand from overseas - with the exception of Los Angeles flights.

On Monday the airline confirmed crew members had been forced to self-isolate after some staff allegedly disregarded physical distancing rules during a layover in Vancouver.

Documents obtained by Checkpoint show increasing unease and fear among flight crew staff about the exemption from isolation or quarantine, and the risk it poses to colleagues and the public.

Air New Zealand is currently operating 16 return international services a week. At the end of May it plans to add three return services a week to Shanghai to that schedule.

For more than a week, Checkpoint has repeatedly asked Air New Zealand, the Ministry of Health and the Director-General of Health Ashley Bloomfield for the number of Air NZ staff who have tested positive for Covid-19. Nobody has been able to provide the information, and Air New Zealand has declined requests for an interview on Checkpoint.

In a statement, an Air NZ spokesperson said the company had been following guidance set down by the Ministry of Health.

"This is expert medical advice for all airlines to follow in New Zealand. If there are general concerns or questions about this advice then that is a matter for the Ministry of Health as they have established these standards.

"Staff who have concerns or questions around this guidance can raise this with the airline in a variety of ways. We have our team of in-house aviation medicine doctors who have been providing detailed advice to staff on reducing the risk of spreading infection and on the MoH guidance. The use of PPE forms part of the MoH guidance."

New Zealand's biggest Covid-19 coronavirus cluster is the Bluff wedding, where the virus has spread to nearly 100 people and killed two, including the groom's father.

The cluster has been officially linked to overseas travel. An Air NZ flight attendant who had just returned from the United States and had already been exposed to Covid-19 was at the wedding reception.

"On 19 March, NZ5 arrived at Auckland from LAX on which three passengers tested positive for Covid-19, at least two crew later tested positive. A crew member from that flight, before testing positive, went down to Bluff to attend a wedding, and now we all know about the 'Bluff cluster'," an Air NZ employee told Checkpoint.

Four days before that, Prime Minister Jacinda Ardern announced compulsory 14-day self-isolation for anyone arriving in New Zealand from anywhere in the world, excluding the Pacific.

Despite the clampdown, Air NZ crew remained exempt at the time and have largely maintained that exemption throughout the pandemic.

On Monday 20 April, Air NZ's Shareholding Minister Grant Robertson told Checkpoint it was his understanding the exemption had continued.

That is despite employees repeatedly raising concerns that the lack of isolation for returning crews was endangering them and other people, Checkpoint has learned.

A letter sent to Air NZ management earlier in April starkly laid out the issue:

"An Air New Zealand flight arrived at Auckland from which three passengers tested positive for Covid-19. One in each class throughout the aircraft. Four crew later tested positive.

"Another crew member from that same flight, before testing positive, attended meetings, then embarked on a four-night tour of duty.

"One of those meetings was attended by yourself. If the 14-day separation rule was in place, that crew member would have remained in self-isolation at home and would not have placed other members of the community or colleagues at risk of Covid-19."

Ministry of Health guidelines exempt aircrew from 14-day stand downs between different flights as long as they appear healthy, but the same letter noted these protocols are minimum guidelines.

Air NZ has sufficient surplus crew to accommodate a mandatory 14-day stand down between flight duties to allow for the virus incubation period.

"Any socially responsible business that goes above and beyond these minimum requirements is helping create an environment where social transmission of Covid-19 is reduced," the letter said.

"We urge you to implement the recommended 14-day stand down between duties."

The letter was not the first time Air NZ management had been asked to tighten the rules.

In March, union representatives wrote to the company leadership:

"Regrettably, yesterday we learned that there are now six Air NZ staff who have tested positive for Covid-19. All of these cases are linked to overseas travel. While we wish to protect these individuals' privacy, it is entirely reasonable to surmise that all cases are cabin crew or pilots.

"We believe it is time to implement a hard 14-day separation rule between all duties, to ensure the risk of crew unintentionally spreading this Covid-19 virus is mitigated.

The letter went on to say it was almost certain there would be further crew infections.

The current cases indicate assumptions made by the Ministry of Health around flight crew protocols must be wrong, as crew are still contracting the virus despite following the recommended procedure.

In an email dated 28 March, it landed on this response to repeated requests for mandatory isolation for all crew:

"We are looking into this. Currently, the advice from our medical team will stand for now - as per the alert level, crew will be required to self isolate in between duties. Air NZ flight attendants are considered to be essential workers and therefore have dispensation to come to work on the provision they fully fit for work."

So at level 4 crew are supposed to stay at home between flying jobs, but unlike any other passengers coming to New Zealand from overseas they can get on another flight with potentially hundreds of other people - within the incubation period for the virus.

The two letters raising these grave concerns were sent after the Bluff wedding on 21 March, but a trail of documents obtained by Checkpoint shows questions were being asked about the crew quarantine exemption as early as January.

In February, the advice remained the same: no quarantine required.