A major fire overnight in the Waikato Hospital laundry is being treated as suspicious as Waikato District Health Board asks wards to conserve laundry.

It took firefighters from seven Hamilton fire trucks to bring the 3am fire at the laundry and services building on the hospital campus an hour-and-a-half to extinguish, Northern Communications Centre shift manager Daniel Nicholson said.

Three fire safety investigators remain at the scene this morning but Nicholson said early reports indicate the fire is suspicious.

One had been on site for six hours and another five-and-a-half trying to establish the cause.

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No one was in the building and there were no injuries but Nicholson said the rear of the building on Hague Rd at the edge of the campus was damaged.

Police and St John were also notified.

Waikato DHB spokeswoman Lydia Aydon said it was too early to know the cause of the fire but that the laundry sustained "quite a bit" of damage including to its wiring, which meant the DHB had to use reserve linen for the 600-bed hospital.

"We are using the extra stock in the linen cupboard and asking wards to conserve laundry."

The laundry is run by Taylor's Laundry who operate laundry services in Auckland where the DHB will get extra linen from this weekend.

Aydon said engineers were on site and assessed the laundry as likely to be usable by Monday.