Security guard pay scandal rocks NGV

Security guards at the National Gallery of Victoria say they are being underpaid and in some cases, paid cash in hand.

Dozens of United Voice security union members rallied outside the gallery on Wednesday demanding that its contractor Business Risks International repay as much as "three million dollars in stolen wages".

"Earlier this month BRI Security was exposed in the media for underpaying gallery guards by almost $20 an hour in some cases," the union said in a statement.

The union says guards report being hired by BRI and then transferred to subcontractors who underpay them, and in some cases paid cash-in-hand.

The NGV said the government is "considering the matter" and would not provide further comment.

BRI declined to comment.

The government ordered an immediate investigation once the allegations came to light and it "will have more to say shortly".

"The investigation by independent auditor KPMG has concluded," a government spokeswoman told AAP.

"The department has just recently met with the security service provider and sought an explanation within one week."