Australia Post has been accused of using computerised letterboxes to illegally spy on its own workers.

Six postal workers at Coogee, in Sydney's east, were accused of falsifying time sheets last year in a case in which management used computer-monitored street letterboxes to collect data.

But Australia Post is trying to side-step criminal proceedings by having them struck down in the Federal Court.

Jim Metcher from the Communications Electrical Plumbing Union says the practice is a breach of state surveillance laws.

"Australia Post are responsible and required to inform their employers if they are monitoring their work performance," he said.

"In this case, Australia Post introduced new systems, computer-based systems, to spy on their workers, and it was found in the evidence that they used this system to ultimately dismiss these employees from their employment."

The union says its court battle could set a national precedent.