Leaders of eight unions with members in the service have been summoned from around the country to attend a meeting at the Public Service Commission's Woden headquarters on Thursday afternoon.

The unionists have been told simply that ''the purpose of the meeting is to canvas the views of interested unions regarding possible changes to the current process for bargaining in the APS.''

The Canberra Times revealed on Wednesday that the pay gap between public servants on the same classifications but different departments might have swollen to $47,000 a year with collective agreements for all 108 agencies to expire on June 30, 2014.

Unlike the past round of talks in 2010 and 2011, the main public sector union, the CPSU, says it will not push for a central agreement.

The union movement's stand on central bargaining crumbled in 2010 in the face of a hardline commitment by the Labor government to agency autonomy.

CPSU National Secretary Nadine Flood recently told The Canberra Times that her union would not push for a single agreement in the new round of talks.

''Our policy position is not to have a single agreement, hasn't been for some years,'' Ms Flood said. ''Reducing pay gaps is an important issue but the issues that are keeping public servants awake at night are job security and pressure conditions if there is a change of government.''

The Public Service Commission was coy on Wednesday about the agenda for Thursday's although a spokeswoman confirmed that commissioner Stephen Sedgwick would not be attending.

''The APSC is not in a position to provide any advice on future bargaining arrangements at this time,'' the spokeswoman said.

The Australian Service Union, representing taxation office workers, has signalled that it would fight any move away from the agency bargaining status quo, telling members it is prepared to take legal action to protect its ability to negotiate directly with tax bosses.

The union's tax officers branch secretary Jeff Lapidos has issued a statement to his members pledging to resist other unions if they tried to pursue a central bargaining position.

''We have pledged to take legal action to protect our right to bargain with the ATO for our pay and conditions,'' Mr Lapidos wrote. The Australian Service Union clashed with other unions in 2010 and was outvoted in the ACTU, which chose to take the ill-fated central bargaining position to the negotiating table.

''We continued to argue our position in favour of bargaining for pay and conditions, agency by agency, as has been the case since 1998,'' Mr Lapidos said.

''The government adopted the essence of ASU's view. The ASU has been able to win pay and conditions in the ATO that are amongst the best in the Australian Public Service.

''Bargaining for the next round of enterprise agreements in the APS will be difficult, no matter who wins government after the election later this year. Money will be tight whoever wins.''