The supermarket giant has denied shelves are bare, but photos posted by the United Workers Union on social media claim some stores across NSW and Victoria have been left with empty shelves.

Woolworths insisted all stores were still receiving regular stock deliveries, but some were getting their goods less frequently than previously scheduled.

No product limits were in place apart from eggs, which had been rationed for some time following bird flu outbreaks in NSW and Victoria.

Woolworths said that a Melbourne distribution centre would also be reopened to help improve the stock levels in Victorian stores.

While the staff are asking for a pay increase, the United Workers Union also wants Woolworths to abolish a performance framework it says is pushing staff to sacrifice safety standards in pursuit of faster work.

At a rally outside a western Sydney distribution centre, workers held up placards with slogans like "we're not robots".

The message was a reference to a framework that allocates them a certain time for a task and then ranks their performance out of 100, something they say puts undue pressure on them and has a negative impact on wellbeing.

The union claimed the strike was already having a huge impact, with "gaping holes" on shelves normally containing household and frozen goods.

The grocer conceded that some Victorian, NSW, and ACT stores had "limited stock flow" but added that extensive contingency plans were being implemented to minimise the impact on customers.

A Woolworths spokeswoman said the company was working hard to sort out the pay dispute.

With AAP.

Image: United Workers Union