That put it above other potential hazards like workplace aggression and violence.
"The systemic effect of this surge will have a significant impact on already fatigued and stressed hospital resources," the report said.
At the height of the outbreak, hundreds of staff - most of whom work at Auckland City Hospital - were off with the virus, meaning people had to fill in in different roles or work extra shifts or overtime.
The union, the Nurses' Organisation, said the risk should have been classed as critical long before now.
Kaiwhakahaere Kerri Nuku said the high risk category should not be temporary.
Nurses had gone the extra mile, some stepping back into the wards after decades behind a desk or working in the community, she said.
There was no plan to give them a break - especially before the busy winter.
"Unless we've got some steady mechanisms in place to allow the nurses to recover, to replenish how we're short then we are just scratching the surface and pretending everything is okay underneath it," she said.
The board had requested a "deep dive" into the problem of fatigue to be presented this month but it had not been completed.
"The team do not have the capacity to undertake the work required at this time," the report said.
