A NZ Steel employee is one of the four Kiwis to contract coronavirus, with staff told about the confirmed case by email and in emergency meetings.
The steel manufacturer's chief executive, Gretta Stephens, told staff the company had been advised of the new case on Thursday morning.
"We are pleased to advise that the affected employee has confirmed that they are not seriously ill and are in isolation at home," Stephens wrote in an email to staff.
NZ Steel has confirmed the email was official and sent by chief executive Stephens.
She said the company was working with the Ministry of Health to handle the situation.
"This includes contacting and working with those employees who have had primary contact with this employee," Stephens said.
"On advice from the Ministry of Health, primary contact is currently defined as direct face to face contact for more than 15 minutes or working in close proximity in a confined area for more than two hours."
NZ Steel, owned by Bluescope Steel, operates the Glenbrook steel mill about 40km south of Auckland.
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The mill employs more than 1000 workers but Stephens said the employee had not been at the mill.
"We can confirm that the affected employee has not been in any plant operating areas or the cafeteria," she said.
She said she would update employees as more information came to hand.
The employee is one of four New Zealanders who have contracted the virus.
The third confirmed case of Covid-19 was an Auckland man in his 40s who had not been travelling overseas.
Director-General of Health Ashley Bloomfield told reporters it was believed to be an instance of transmission between family members.
This is the first case of person-to-person transmission inside New Zealand's borders and Bloomfield did not expect it to be the last.
The partner of the now-infected man had returned home from Iran on Qatar Airlines flight QR0920 from Doha to Auckland.
That person did so a week before the Government imposed travel restrictions on Iran.
Two of the four people with the virus are a husband and wife who recently returned from Italy. The wife was confirmed as New Zealand's second case this week - and the husband was confirmed as the fourth case this morning.
The couple have been in isolation at their Auckland home with their two high-school aged children. Their schools, Westlake Boys' and Girls' high schools, have been alerted.
The family have been subject to social media bullying.
The Herald revealed yesterday the woman had been seen at the Westgate Medical Centre, where protocols had been followed - but a staff member told the Herald they were upset they had only found out about the confirmed case in the news.
The woman insisted yesterday she had tried to follow all medical advice, including visits to two medical centres.
"People assume we are these monsters that have all these symptoms out in public infecting people. We've had very, very, very few symptoms," the woman told Stuff.
"Can you imagine my children going back to school at this point? There are already people pulling their kids out of school when they've been told really clearly that there is absolutely zero threat to their children."
Both schools the children attend, Westlake Boys' and Girls' high schools, have been alerted.
Health officials were yesterday also contacting 100 passengers who travelled on two domestic flights between Palmerston North and Auckland with the infected woman before she was diagnosed.
But the Ministry of Health maintains the chance of widespread community outbreak remains low.
The ministry confirmed the fourth case this morning.
"Contact tracing is under way and close contacts are already in self-isolation," the Ministry of Health said in a statement.
It said further details would be provided later today as well as any results of other testing around the country.