The Fair Work Commission has begun to hear arguments in the ATO’s hot-desking case.

The Australian Taxation Office (ATO) moved to expand hot-desking – a practice in which staff use whatever desk is available each day, and keep their possessions in a locker at night - after a new industrial deal had been reached, the CPSU says.

The union argues staff thought the workplace deal they supported early this year would include less hot-desking.

When finally ended their prolonged bargaining dispute over pay and conditions in June, three proposed agreements that removed or changed hot-desking clauses had been rejected.

But the ATO then revealed plans for hot-desks at a new Gosford office without telling union or staff, according to the unions.

“There's also no indication or even suggestion that the employees were advised that a decision had been made to hot-desk at Gosford,” said Fiona Knowles, representing the Australian Services Union.

Community and Public Sector Union spokesperson Aiden Nash said unions had been clear that they opposed hot-desking in negotiations.

ATO representative John Snaden said the agency had been clear.

“Gosford was always going to be a shared accommodation site,” he said.

“The intention for Gosford was clearly and consistently put from as early as December last year and the unions can't pretend that they were blindsided because they never got some formal notice that the ATO executive had ticked off on it.”

He said staff had accepted the arrangement by supporting the broader workplace deal.

Mr Sladen said the ATO only tried to remove hot-desking clauses in the enterprise agreement when they breached the public service commission's workplace bargaining policy.

The Fair Work Commission has not yet revealed its decision on the dispute.