Health authorities are trying to confirm how a nurse at Middlemore Hospital's emergency department caught Covid-19.
The nurse is fully vaccinated, was wearing appropriate PPE whilst at work, was asymptomatic and took appropriate precautions.
The Ministry of Health said they returned a positive result during routine testing.
The case is not yet being included in the current outbreak tally. The ministry said the public health risk is low.
But the hospital does have links to current cases - the index case was transferred there on 16 August; and the weekend before lockdown a patient, who later tested positive, spent up to four hours in the ED waiting room while they were believed to be infectious but left before seeing a doctor.
NZ Nurses Organisation kaiwhakahaere Kerri Nuku told Checkpoint there was an ongoing investigation into the case.
She said it was worrying not knowing how the nurse had caught the virus.
"It's completely frustrating."
She said even before the pandemic, the health system was in a "significant amount of crisis".
"We had already talked about the significant understaffing and for that reason many of our members had voted to go on strike to make it really clear to the government, the pressure that is currently within the system".
She said such outbreaks put the system under enormous pressure.
The frontline nurses at Covid-19 testing and vaccination centres were exhausted, she said.
"We've got a tired workforce, we've got a frustrated workforce - this not knowing how do we protect ourselves and be safe at work is certainly wearing on a few people."
Covid-19 Response Minister Chris Hipkins is seeking advice on whether to make vaccines mandatory for frontline health workers who come into contact with confirmed Covid-19 cases.
Currently only border workers - people working at managed isolation and quarantine (MIQ) facilities, ports and airports are required to be vaccinated.
She said the union was not yet consulted with about mandatory vaccination for health workers.
"Our own organisation is going to be having some discussions about where our position is and what we'd encourage members to do, but that's not something that we've actually fully discussed yet."
