The Bay of Plenty district health board is facing a bill of more than $100,000 after underpaying some staff for the past six years.
The board attempted to have a collective agreement rectified to reflect payment of time and a half, rather than double time, for Medical Radiation Technologists (MRT) called in to work after hours.
But in a recently released ruling the Employment Relations Authority declined the bid.
The authority ruled the board had to pay MRT staff double time, which had been agreed in their collective agreement, when they worked between 8pm and 8am during the week and on weekends and public holidays.
The board was also ordered to pay backpay for the period starting March 2007.
The wage error was picked up by a new staff member called to work late one Friday night in 2011.
She realised her pay was not as much as at her previous job and checked the contract.
"On closer inspection of this clause I believe all evening callouts are being paid incorrectly and any call between the hours of 2000-0800 should be paid at T2 [double time]," she wrote in an email to the payroll administrator.
Some backpay was paid to some staff before the board revised its view of its obligations under the contract's clause.