The Public Service Association has asked the Office of the Auditor-General to investigate the Government's use of ministerial advisers in government departments.
The part-time "private purchasing advisers" are paid for by the departments, but answer to ministers.
Association national secretary Brenda Pilott says the appointments are unlawful.
Ms Pilott says the advisers should be paid for out of ministers' budgets and that the arrangement breaches the principle of political neutrality.
Ms Pilott says lawyers, academics and senior public servants have approached the association to express their concern about the appointments.
The association says use of the advisers suggests the Government does not trust public servants.
The Labour Party has accused the Government of appointing spies in government departments and of bypassing the usual rules on appointing political advisers to ministerial offices.
Labour Party MP Chris Hipkins says use of the advisers allows ministers to increase their staff without increasing their budgets.
Mr Hipkins says this represents a radical change from the way ministers' offices have been staffed in the past.
However, the State Services Commission says it does not believe the advisers are unlawful.
Finance Minister Bill English says the "private purchasing advisers" are helping ministers to trim departments' budgets.
In a series of answers to written questions, some ministers have confirmed that they have hired purchase advisers, who are paid by their departments.
Mr English says ministers need people alongside them who are not part of the public service but understand state sector mechanisms.
In a letter to ministers, Mr English said the purchase advisers would advise ministers on whether what their departments were doing was aligned with the Government's priorities.
He said the purchase adviser would be accountable to the minister but would work mainly from his or her own base, rather than being formally set up in a minister's office.
