Costco, the US retail giant, has opened its first stores Sydney and Canberra.

The openings herald a major shift in the Australian market, intensifying competition in an already challenging retail environment.

Costco is a members-only retailer that provides business and private members with low-cost and bulk-packaged products from a range of categories including groceries, clothing and homewares.

The retailer is known for selling bulk imported products and popular local supplier brands for at least 10 to 20 per cent cheaper than its rivals.

Costco's first Australian store was opened in Melbourne's Docklands precinct in 2009, generating immediate success with sales of $9 million within the first two weeks.

Costco hopes to replicate that success in its Sydney and Canberra stores.

The new Sydney store is in Auburn, in Sydney's west.