The dispute over the office fit-out craze has sucked in a second trade union with the Community and Public Sector Union and the Australian Services Union now both opposing the ATO's plans for its workforce.

The two unions insist the Tax Office is breaching its workplace agreement with its hot-desking plans which have not gone down well with the ATO's rank-and-file, they say.

But the Tax Office disagrees, saying its "activity-based working" programs were proving popular with its public servants.

Hot-desking - an office plan where staff find a new desk each day and pack their work into lockers before going home - has been widely adopted in the private sector by firms looking to reduce wasted space and build teamwork.

But the trend has caused frustration for many employees who say it disrupts their work and poses health and safety problems.

The ASU said last week that Tax Office staff were not consulted about the trial and that meetings with senior ATO executives to try to resolve the disagreement had proved fruitless.

Union official Jeff Lapidos told his members he believed that, while the ATO had only earmarked two sites for hot-desking so far, it wanted to reserve the right to introduce the practice at any of its worksites around the country, at any time.

"So hot-desking will be coming to you, sooner or later, unless the ASU is able to stop it," Mr Lapidos wrote in his bulletin.

Mr Lapidos has previously said that tax officials needed workstations set up for their health needs to prevent injuries from repetitive movements.

The union says ATO staff also wanted to put up photos of family or have other personal affects around their desk to boost morale, which they could not do under a hot-desking regime.

The Tax Office would not comment specifically on the dispute, but a spokeswoman said on Tuesday that employees were lining-up for its "activity-based working" program.

"Our pilot site for activity-based working has a waiting list of staff eager to work in the environment and we have many staff across other locations asking for similar work spaces to be installed so we have the type of professional and modern workplaces they see in a large and growing number of other organisations," she said.

The CPSU, for its part, says it fought off an attempt by the ATO to introduce hot-desking in 2013 and the latest push was in breach of the office's new enterprise agreement which only requires employees "regularly engaged in fieldwork" to share desks

The union says the hot-desking trial in the ATO's office in Melbourne's docklands last year was voluntary but the program in its controversial new building at Gosford in NSW was mandatory.

Only a small number of about 400 public servants slated to work from the new office in the central coast town could be categorised as "employees regularly engaged in fieldwork", according to the CPSU, and the union wants the ATO to stop fitout of the Gosford office until the dispute is resolved