Thousands of Australia Post workers have stopped work, and plan to take industrial action until Christmas eve.

Workers also plan to rally outside Australia Post's Sydney headquarters today, demanding better pay and conditions.

It is the second strike in a week for Australia Post workers, after 20,000 employees walked off the job last Wednesday, disrupting mail services nationwide.

A defiant Australia Post said it was taking on casual workers to lessen the impact of the strike.

The union representing postal workers wants a new workplace agreement that protects penalty shifts and pay, and improves workplace safety.

The Communications, Electrical, Plumbing Union (CEPU) says the industrial action is the last resort following years of failed negotiations.

"We expect that there will be delays in the delivery of mail; we certainly apologise for that," union president Ed Husic told ABC radio.

"Our move is not going to be popular with a lot of people but at the same time we are trying to represent mums and dads."

CEPU's NSW secretary, Jim Metcher, told The Australian: "It's about putting influence on Australia Post."

Mr Metcher denied the action was an illegal wildcat strike.

"Fair Work Australia has approved all of the action to be taken by the workers. It's protected under law."

Australia Post said a contingency plan has been put in place.

"This is not negotiating in good faith. This is strong-arm tactics from them," company spokesman Alex Twomey said.

Fewer than half the union's members had voted for the strike and those who had not would be reluctant to give up pay and substantial overtime at this time of year, he said. That meant only about a quarter of the workforce would be absent, he said.

"Really what we're trying to do is cover that 25 per cent (of staff that will strike).

"We've got ... Christmas casuals and we have beefed up those numbers."

- with AAP and The Australian