A second man has been arrested after a spate of suspicious fires in Hamilton early on Friday morning.
The man, aged 19, faces a charge of arson.
Police say six fires were lit, the most serious being in a wheelie bin at Braemar Hospital that caused considerable damage and resulted in the evacuation of 20 patients.
A process worker, 22, was arrested on Friday morning and faces six charges of arson.
The blaze at Braemar Hospital in Frankton started in a wheelie bin at 2.42am and quickly spread into the air conditioning system and the roof of the building.
Of the 32 patients in the hospital, 20 were moved out of the building to other hospitals.
Waikato Assistant fire commander Roy Breeze said the sprinkler system stopped the fire spreading further, but the blaze had burnt through gas and water mains feeding the building, and gas was leaking.
He said more than 30 firefighters fought the fire.
"We really could have had quite a major incident," he said. "It was quite major in the sense that 20 people who had not long undergone surgery were wheeled out into the carpark at thre o'clock in the morning. It's just not a good situation."
Mr Breeze estimated the damage would run to millions of dollars. A large x-ray machine and related equipment was drenched by the sprinkler system.
Police say the blaze was one of fire fires in the city on Friday morning. The other fires were in a skip at Hamilton Girls High School, a phone box, a car and a pile of garden waste.