A nurse at Waitemata DHB - where three staff have contracted Covid-19 - says its use of PPE has been inadequate.
The third Covid-19 case was confirmed today at Waitakere Hospital, with two other cases in the past several days.
The source of infection has not been confirmed, though the staff cared for St Margarets rest home residents with Covid-19.
The DHB told the Health Ministry all staff wore PPE when caring for them, and that nurses had access to full PPE and wore it at all times
The nurse said it "beggars belief" the PPE would fail.
Director of Public Health Dr Caroline McElnay said the DHB was investigating.
"They really need to investigate this fully to make sure there are no possible breaches of PPE."
PPE was not "completely foolproof", government minister Grant Robertson said.
The nurse said it "wouldn't be 100 percent foolproof, but it certainly would be very, very effective" and it was hard to believe the PPE itself would fail.
The DHB has stood down 57 staff over the risk of infection.
It said it aimed to have an audit done by next week by someone from within the DHB.
The nurse, who spoke on condition of anonymity to RNZ, scoffed at that: "That's like an organisation using their own auditors that are paid by them, to audit them."
The DHB's communication about PPE and its use and fitting of full-filter N95 masks, had been poor, she said.
"The N95 mask, none of the even senior staff seemed to know [a month ago] there even was such a mask, and that those N95 masks are required to be tested to each individual person."
Another source said there are current, ongoing problems with fitting the masks for staff across different areas of the DHBs' hospitals at Waitakere and North Shore.
The nurse said that more recently better information began coming out, but in fits and starts, and by mid-April there was no consensus on mask use, and debate on PPE use was still going when the resthome residents were being admitted.
She did not have confidence in the DHB so had done her own research into masks.
"It strikes me as negligent, but not strange.
"Because in my experience, particularly at Waitemata DHB, is that they are ill-prepared, their communication is not consistent."
She had heard of another DHB, in the South Island, that also had poor PPE practice, she said.
The investigation into the three staff contracting Covid had to be independent and not left up to the DHB, she said.
"So what they need to do is ... independently consult with the people that were directly involved.
"They need to speak with the nurses that were affected, and to guarantee them anonymity and immunity from any negative impact that the DHB might have against them.
"There needs to be more accountability and independent investigation."