Australians have been able to make cheap phone calls to landlines and mobiles using a new Google Gmail feature that launched this week for US and Canadian users.

Australians who have their Gmail language set to "English US" are able to use the Google Voice plug-in for Gmail to make calls using the microphone on their computer. The service throws down a significant challenge to Skype and traditional phone service operators.

A screen grab of the new Google Gmail free phone call service accessed by an Australia user.

Users start with $US0.10 credit, which can be topped up using Australian credit cards.

Calls and texts to US and Canadian numbers are free with very low rates for calls to phones in other countries. Calls to Australian numbers are charged at the per-minute rate of $US0.02 and $US0.14 for landline and mobile calls, respectively.