Thousands of Trade Me users have contracted a computer virus through the popular auction site.

The virus has caused significant computer problems and some suspect it has stolen their credit card details.

Trade Me spokesman Paul Ford said the company estimated thousands of users had been affected. Police had been informed and the company was reviewing its procedures.

Ford said users' computers contracted the virus through a malicious advertisement supposedly from Lonely Planet. Trade Me accepted the advertisement online from someone who claimed to represent the travel book company.

The virus struck on Thursday morning and was tracked by programmers by midday Friday. The company then removed the advertisement and began telling users what had happened.

Ford said the virus could detect old browsers or computer systems combined with out-of-date anti-virus software. When it detected the right combination, it told users they had a virus and offered to remove it by downloading a programme. Accepting the invitation then installed the virus.

"It is the first time we have been targeted. We take it very seriously."

The freephone number for Trade Me customers affected by fraud is 0800 334 332.