Enough New Zealand computers are infected by malicious software to put one in every house in Hamilton, a survey has found.

Statistics New Zealand's survey of internet service providers (ISPs), released today, found 55,000 computers had been compromised by malicious software - as many infected computers as there are households in Hamilton.

The so-called zombie or slave computers can be used to send out spam, viruses, or engage in cyber-warfare without their owners even knowing.

About half the ISPs in New Zealand monitor their networks for compromised security activity, with four per cent of their connections showing signs of such activity.

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NetSafe executive Martin Cocker said it was good to see a growing number of ISPs monitoring internet traffic for activity such as cyber attacks.

"The bad news is that these results show that a very significant number of Kiwi homes are likely to be using computer connections that are unsafe, potentially putting their data and other users at risk," he said.

Infected computers may still seem to function normally and show no outward signs of being compromised.

"Malware can be used to steal your information including credit card details, destroy data, drain your internet data allowance, or attack other computers," Mr Cocker said.

"The good news is that it is relatively easy to put basic security measures in place to protect your computer."

People could protect computers by installing anti-virus and anti-spyware software, using an active firewall, and setting software to automatically update.