New Zealand government websites may be knocked offline for days or weeks or defaced by overseas hackers if Megaupload.com founder Kim Dotcom is extradited to the United States, a company specialising in the security of information technology systems has warned.

Activists associated with hacking group Anonymous vented their outrage at Dotcom's arrest last week by launching "denial-of-service" attacks that temporarily blocked access to the websites of the US' FBI, Justice Department and recording label Universal Music Group.

Aura Information Security managing director Andy Prow believed New Zealand government sites might now be in their sights and warned some might be easy targets because of their relatively small size.

Prow said that since Dotcom's arrest, two of its government clients had experienced some fall-out from "denial of service" attacks, which bombard websites with traffic, usually from networks of malware-infected computers. There was a chance hackers could go further, hack into sites and do "far worse", Prow said.

"Many New Zealand websites are not secure enough to survive a dedicated attack and very few organisations have an effective incident response plan should they be targeted. I'd see this as the 'Metservice's heavy storm warning' nothing may come of it, but if it does, make sure you have got the hatches battened down."

Anonymous briefly knocked Parliament's website offline last May in protest at changes to the "Skynet"copyright law that allows the Copyright Tribunal to fine internet pirates $15,000 after three written warnings from copyright holders. The longest denial-of-service attack Prow had seen had taken a site down for 10 days.

There is some speculation charges against Dotcom and his associates may be settled without going to trial in the US, given the likely complexity of the case and that United States' authorities appeared to have achieved many of their objectives by shutting down the file-sharing service, associating illegal file-sharing with serious organised crime, and persuading other file-sharing services to tighten up their own policies to discourage piracy by members.