A West Australian Coles supermarket manager has been slammed by the retail workers union after asking staff to work for four hours on a Sunday night without pay, but plenty of pizza.

The manager of the store posted a note in the shop, asking his 65-people workforce "to give me 4h free labour" to clear a backlog of stock in a storeroom.

No pay, but pizza: The note posted at a WA Coles store. Credit:ABC News

"Yes, it is a big ask and I would not be requesting this unless it was absolutely necessary."

The reason, so the note said, was an "enormous amount of stock out in the back".