Internet provider iiNet has had a major privacy breach after more than 30,000 customer passwords were hacked.

A Twitter user with the handle CyberWar_News posted a screenshot on the social network on Monday, showing an anonymous hacker was selling or trading the database of Westnet, a WA-based internet service provider that was acquired by iiNet in 2008.

An anonymous hacker has offered Westnet customer details up for sale. Credit:Twitter

"There is a shitton [sic] of data inside the database to where I haven't even bothered to dump this whole thing but there is a lot of valuable data inside like cleartext, password etc...," the screenshot said.

"For those interested can contact me on via jabber below."