Wellington, Dec 18 NZPA - Parts of the Waikato District Health Board (DHB) computer system, struck down by a virus early yesterday, are back on line today -- but the whole system is not likely to be operating again until Monday.

The Conficker virus infected 3000 computers logged on to the DHB's network early yesterday morning and forced a complete shut down.

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

Waikato Health chief operating officer Jan Adams said technical staff had made good progress over the afternoon.

"We do have some of our critical clinical areas now back online and we'll be bringing the rest of them up over the next 24 to 48 hours.

"So we're on target to have theatres and all our out patient clinics working as normal on Monday."

The hospital had remained fully operational, but people were still advised to visit their GP first and non-urgent patients to stay away from emergency departments, she told NZPA.

That would be reassessed tomorrow when the emergency management team met.

It was not yet known how the virus entered the system, she said.

"Once we've actually got everything clean, back up and running and we're normal, we will then go into post-event review and actually do a full analysis of how did this occur."

Information systems staff were monitoring the system closely, upgrading it and cleaning it to make sure it did not get reinfected, she said.

"From our view point obviously it's a massive, massive inconvenience, but our patients are continuing to receive good care."

Manual practices were being used to do what computers would normally do.

DHB spokeswoman Mary Anne Gill said the switchboard, emergency department at Waikato Hospital and operation centre were back online.

It was hoped the laboratory, radiology, emergency department in Thames, delivery suite, pharmacy and medical records would also be back up and running today.

"That's a great result really for what's happened," Ms Gill said.

"It's been a massive effort by everybody... We've had 80 to 90 of our information services staff who have not slept for nearly 48 hours."

Those staff would be sent home tonight to sleep and would come back tomorrow to sort the rest of it out over the weekend, she said.

The public had also been "fantastic" at staying away during the crisis.

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.