The union argued the situation had been "overstated": It was a normal busy day with normal staffing, and the health board had hours to act before the strike.
A letter from the health board's lawyer to the union dated November 24 called the staffing during the two-hour strike that morning "a highly dangerous situation".
"There was a clinical need for four midwives, but only two were on duty and/or available. Fortunately, this did not result in any adverse outcomes on this occasion, but the situation was completely unacceptable."
The letter said the union had made good faith agreements to ensure the hospital could still deliver any life preserving service during a strike. If the health board could not cover the period without the help of union members, they would still work the shift.
It raised concern about staffing levels for other strikes and said that if that union could not provide the minimum agreed on-call staffing for a shift it should stop the strike.
"To refuse to do so would not only be a breach of good faith, but also jeopardise the lives of mothers and babies."
In a response letter, union co-leader (midwifery) Caroline Conroy said the situation in Whakatāne was "overstated".
"I have spoken to the midwives on duty and they refer to it 'as a normal busy day'."
She said the duty manager was notified at 7am about the busy workload but took no action to improve the staffing levels ahead of the 10am strike.
"If as you suggest that four midwives were needed then I would question why action was not taken in the three hours prior to the strike to ensure this occurred."
During the two-hour strike a registered nurse helped on the ward and a nurse manager arrived to answer the door, so both midwives continued to work normally.
"The midwives are delighted that the staffing levels that they experience every day have finally come to the attention of senior management in the organisation and are looking forward to the DHB now improving them."
The union and the district health board are involved in a long-running pay dispute.
Last week midwives in the union began a series of two-hour work stoppages affecting Tauranga and Whakatāne hospitals in the Bay of Plenty.
The industrial action was planned to continue until December 5.