They planned to challenge the decision and that would include appealing to relevant ministers.
Seven nurses caught Covid-19 after patients from St Margaret's rest home moved to the hospital.
One nurse had to be cared for in a high-dependency unit.
The Resident Doctors' Association national secretary, Deborah Powell, said hospital workers should feel safe doing their jobs and she could not understand why WorkSafe was not investigating.
"One of these nurses ended up in a high-dependency unit she was so ill. Family members of nurses got Covid as well. It was a serious incident."
The DHB's investigation was full of gaps, including only looking at the first three nurses to get the virus, she said.
"We wouldn't be asking the forestry industry to investigate the forestry industry for forestry deaths. We don't do that, WorkSafe investigates because we need to have the sureity that it is independent," Powell said.
The Nurses Organisation Kaiwhakahaere Kerri Nuku said the workers were exposed to a serious hazard doing their job - and if was not fair they were not being investigated just because it happened in the health sector.
"Certainly there needs to be internal investigations but we need some external checks and balances on this," she said.
Unions representing cleaners, administration staff and senior doctors were also challenging Worksafe's decision.
A WorkSafe spokesperson said it had reviewed reports from both the Waitākere DHB and the Auckland Regional Public Health Service and was satisfied remedial action had been taken.
