FAMILIES can save up to $80 on a trolley of groceries simply by switching supermarkets and weaning themselves off established brands.

A nationwide study has found that Woolworths is consistently cheaper than Coles while discount German chain Aldi beats the duopoly hands down.

Consumer advocate group CHOICE bought 33 common items at 110 Coles, Woolworths and Aldi supermarkets
across Australia.

A basket of leading brand products at Coles cost an average $170.54, compared to a similar basket of Aldi own-brand products at just $91.76.

Woolworths was only marginally lower than Coles, however, coming in at $168.74.

In Sydney, Coles, Woolworths and Aldi stores were tested in Bondi Junction, Mt Druitt, Hornsby and Liverpool.

"The big supermarkets make a lot of very loud claims about value, but it's clear Aldi is Australia's cheapest supermarket and it is forcing the big two to compete on price," Mr Godfrey said.

The extent to which Aldi has changed the supermarket landscape since opening its first Australian store in 2001 is evident in states such as Tasmania and the Northern Territory, which have no Aldi presence recorded the most expensive baskets.

Mr Godfrey said Aldi's recent push west was likely a key factor in South Australia and Western Australia recording the cheapest leading brand basket prices, at about $168.00.

However, when it comes to using local ingredient, Aldi drops to the bottom.

CHOICE found of the 28 food items bought in each of the three retailers, country of origin labels showed that Coles had 22 containing at least some Australian ingredients, Woolworths had 21 and Aldi only 15.