Health Minister Jim Wells is conducting a review into the employment of 58 trade union officials in his department at the cost of almost £2m annually.
Most of the 12 Stormont departments employ trade union officials but none comes close to the Department of Health.
Only the Office of the First and deputy First Minister (OFMDFM) and the Department for Culture, Arts and Leisure (DCAL) did not employ any union officials last year.
Nipsa has defended the arrangement as good value for money. The total wage bill last year for employing trade union officials in all the Stormont departments was more than £2.6m. DUP MLA Trevor Clarke uncovered these figures through a series of Assembly questions.
He said he was left shocked by the data and welcomed the commitment of the Health Minister to review the practice.
"Whilst the unions are threatening strike action, it is notable that the Department of Health paid £1.8million during the 2013/14 financial year to finance the work of 58 trade union officials," he said.
"I welcome the Minister's commitment to review the level of spend in this area. Unions have a very useful role. I'm not opposed to them but most people will expect that the trade unions' membership fees should fund the organisations rather than money which could be used on the frontline.
"To put these figures in context, £1million would fund 35 full-time Band 5 nurses or almost 72,000 hours of domiciliary care. It would fund around 100 hip procedures provide over 1,700 weeks in a nursing home for an elderly person."
Many of the trade union members employed by Stormont are from Nipsa, whose spokesman Brian Campfield insisted are value for money.
He explained that employees from the public sector were being released to work along with management to facilitate good industrial relations so the business needs of the public sector are met in the "most efficient way possible". "The evidence states that this needs to be looked at as an investment," he said.
"It is an investment not a cost because what it does is facilitates engagement between employees and employers, and ensures that a whole range of initiatives that employers want progressed is done in as smooth a way as possible."
When asked why members of trade unions did not fund the posts, Mr Campfield pointed out that many of their members are in low paid jobs. "Their dues to us are out of their hard-earned wages and we are not in the business of subsidising government," he said.
Health: £1.8m to pay for 58 officials
Finance & Personnel: £187,000 for five
Social Development: £184,116 for equivalent of 5.8 reps
Justice: £132,271 for three
Agriculture: £123,692.72 for four
Employment & Learning: £92,867.24 for five
Education: £67,025 on equivalent of 2.5 reps
Environment: £31,304 on one