The highly pathogenic H7N3 strain of bird flu has been detected on five farms in Victoria's state's southwest, with more than half a million chickens euthanised in an effort to stop the spread of the disease.

Coles customers in every state and territory can now only buy a maximum of two egg cartons, with the exception of Western Australia.

Coles says the limit is temporary but has not revealed how long it is expected to last.

"We are working closely with all of our suppliers to ensure eggs remain available for our customers, and we are providing support to the industry in responding to the Avian Flu cases in Victoria," a Coles spokesperson said.

Woolworths and Aldi do not have any limits on egg purchases, and no other national stores have publicised limits.

Over the past seven weeks, the virus has been detected on five farms linked to Farm Pride, Avgo, Surf Coast Eggs and another business.

The affected farms in Meredith, Lethbridge and Terang have been placed into lockdown, with at least 580,000 birds euthanised.

With AAP.