Wellington, Dec 18 NZPA - The Waikato District Health Board (DHB) computer system, struck down by a virus early yesterday, could continue to cripple the system for the weekend.

The Conficker virus was spotted about 2am yesterday, infecting 3000 computers logged on to the DHB's network and forcing a complete shut down.

The DHB had hoped to restart computers in key locations, including the emergency department and intensive care unit, this morning but now said today's original midday target was unrealistic.

"We're planning now to be out of action for the rest of the day, and in fact our emergency management team is planning that we may be down for the whole weekend," DHB spokeswoman Mary Anne Gill said.

"We've got connectivity between Bay of Plenty District Health Board and us, we run a joint diagnostic system, so that's up and running. We've also isolated an area that is virus-free, and we're doing all the testing there."

The shutdown had affected hospitals' radiology and lab testing services, and the DHB had asked all non-urgent patients to stay away.

Hamilton, Taumarunui, Te Kuiti, Tokoroa and Thames hospitals, as well other facilities in Te Awamutu and Morrinsville, had all been affected.

Ms Gill said patient safety was paramount and the outage would not stop their care.

"We need to stop and say `look, computers don't diagnose and treat patients, doctors and nurses do that', and that's what our doctors and nurses are doing."

Ms Gill said the DHB had a lot of staff coming back to work today and they needed to set their computers to the "ctrl-alt-delete" state.

"That's how we treat the virus, we need every single one. If there's one rogue PC turned off that we don't know about, then it doesn't work."

It was not yet known how the virus found its way into the system, but all computers were fully up to date with anti-virus software, she said.

Conficker is a "computer worm" which targets Microsoft's Windows operating system, using flaws in the software to co-opt machines, allowing a remote user to take control.

A New York Times report said the virus had affected millions of computers in more than 200 countries.