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.
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.