On Monday, only Timaru Hospital said it had no record of nurses turning down shifts.
Nurses Organisation chief executive Paul Goulter said the union only had anecdotal evidence to rely on, and nurses were telling it there were shifts being left vacant and it was having an impact.
However, he conceded it was impossible to measure.
He believed Te Whatu Ora Health NZ managers were simply not asking nurses to do additional shifts for the week in a "common sense" approach to the action.
It was still worth it to draw attention to the frustration of the nurses, and it appeared Te Whatu Ora had taken note, he said.
RNZ has sought comment from Te Whatu Ora.
Some health staff spoken to by RNZ said there were so many vacancies - both long term and because of illness or leave - it would be hard to tell, day by day, whether there was any difference because of nurse refusals.
Waikato Hospital said it was down 14 per cent of its nurses at the start of the week, comparable to levels in the Covid-19 outbreak.
RNZ asked all of the Te Whatu Ora districts (former DHBs) about how busy they had been.
Waikato, Greymouth, Whanganui and Wairarapa said they were either full or over capacity at the start of the week.
Taranaki said it had "very high" occupancy with several full wards, while Wellington Regional Hospital said it had frequently been close to 100 per cent full.
The union wants the ability to negotiate future payments for working extra shifts as the winter bonus given to nurses in the very busy Covid and flu season comes to an end.
Current arrangements differ by district.
Te Whatu Ora initially said, in a letter to the union, that the action this week likely constituted an unlawful strike.
However, it appeared to have softened its stance, with many of the responses from hospitals to RNZ saying nurses had the right to say no to extra work.