More than 900,000 hours of nursing in Grampian were provided by bank staff last year.
The total amounts to 200,000 more hours than levels recorded three years ago.
Colin Poolman, professional officer for Royal College of Nursing Scotland, said over-reliance on a pool of temporary workers was bad for organisation of the service.
But he acknowledged the bank did offer a valuable “back-up” during periods of heightened activity, such as a noro-virus outbreak, or during holiday seasons.
He said: “Some nurses work the bank and nothing else and others add to their part-time contracts to give them full-time hours.
“Others are full-time nurses who want to increase their income. We see that more as things are tight and people are doing it out of necessity rather than anything else.”
He warned staff were at risk of breaching guidelines on hours by working just one extra 11-hour shift in addition to their regular full-time rota.
“It is not something we want our members to do, but often it is out of necessity,” he added.
“We would encourage them to work within working time regulations.”
The nursing bank was introduced by NHS Scotland to avoid costly overtime payments.
Banks staff receive their regular rate of pay, with some entitled to holiday leave.
Cutting the use of temporary staff will account for around one third of the £19.7m in total savings to be made by NHS Grampian.
A board spokeswoman said that it had received £2.7m in extra funding from the Scottish Government to compensate for a poor funding settlement, and that the money would be used to hire full-time nursing staff.
Scottish Conservative health spokesman Jackson Carlaw said: “I hope the Scottish government will listen to the concerns of the Royal College of Nursing and take immediate action to resolve this problem.”
Richard Baker MSP (Labour, North East) said: “There is no doubt our local health board faces real challenges in recruiting staff to Aberdeen due to the high costs of living here.
“This is why Scottish ministers must engage properly on the issue of an Aberdeen weighting for the salaries of local workers and think again about imposing huge cuts on NHS Grampian’s staffing budgets.”