About 400 Centrelink customer service centres around the country are bracing for chaos on Monday morning after the national welfare agency unilaterally changed its opening time from 8am to 8.30am without informing clients.

The move, which the Community and Public Sector Union calls a deliberate tactic to stall current enterprise bargaining negotiations, was announced to some staff via email yesterday. But there was no consultation with welfare recipients, with customer service advisers frantically phoning clients this morning to alter appointment times.

Current arrangements mandate that Centrelink customer service centres are open from 8am to 5pm. Many welfare recipients line up outside before then in order to have their claims processed quickly.

The 5pm closing time under the new regime would remain the same.

A spokesperson for Human Services Minister Tanya Plibersek says the changes had been forced on the body because of extra work associated with natural disasters, including the Queensland floods and cyclone Yasi. The spokesperson said there had been 500,000 claims for assistance in the last month, and normal hours would be resumed once the backlog had been cleared.

But the union rejected that interpretation. “Staff are gobsmacked,” deputy national CPSU president Lisa Newman told Crikey. “Workers have not been consulted, nor has Centrelink consulted with clients. This is a recipe for chaos and we’re calling for the decision to be put on hold until we have all the details.”

The union, which represents over half of Centrelink’s giant workforce of 20,000, but whose enterprise bargaining agreements impact on everyone, said it had lodged a formal dispute with Centrelink this morning and will proceed to Fair Work Australia on Monday if their demands are not immediately met.

It said many offices have been left in the dark over the changes, with advisers unaware that opening hours had been slashed when contacted by interested parties. The broader welfare rights and representative sector, which works closely with Centrelink as part of its contracting arrangements, have also expressed dismay.

The decision could place Centrelink staff at an automatic disadvantage in the forthcoming wage negotiations, as the government body moves to cut costs and drive job-seekers to the internet to avoid costly face-to-face consultations.

Crikey contacted Centrelink’s media office as our deadline approached and we will attach their statement to this story when we receive it.