Australians are getting less bang for their buck at the grocery store.

Many grocery items on New Zealand shelves are imported from Australia.

New figures there show the size of some popular items have shrunk by as much as 30 per cent - but their cost hasn't fallen.

Tom Godfrey, from Australian consumer watchdog Choice, said deodorants and chocolate among the worst offenders.

"It's an all to common marketing trick to reduce the amount of food you get in the pack while maintaining the same price, and it turns out that it's rife and that it's been going on for a very long time."

He said it's best to shop around for value

"We're paying a lot more for our much loved products as a defence to this what you need to do is look at the unit price, that price per hundred grams and that price per kilogram."