Industrial action could be sparked if thousands of public servants are asked to work nine minutes longer a day.

Asking employees to knock off at 5pm rather than 4.51pm and have other conditions reduced alongside negligible pay rise offers is expected to prompt calls for industrial action at the Australian Tax Office.

Unions have revealed the ATO and Defence Department have floated the idea of longer working hours in consultations with their staff in the lead-up to official bargaining on agreements, while Geoscience Australia had reportedly brought up the prospect during bargaining.

An extra nine minutes daily would take the average working week at the ATO from 36 hours and 45 minutes to 37.5 hours. The change would add another three hours at work each month or 39 hours a year.