The buildings, on the hill above the town, have been a target for vandals since the hospital was relocated to Takaro Park in 2000.
The property is being converted into an accommodation complex and a 21-unit motel is to open soon in the former maternity annexe.
Project manager David Liu said it was the worst case of vandalism there had been at the hospital.
Those responsible had gained access to the interior of the main hospital building and the damage, to both windows and lights, would cost about $4000 or $5000 to repair, he said.
Mr Liu lives on the site but did not hear anything suspicious on Monday night.
He and two workers discovered the damage at 8am yesterday. He would now consider installing surveillance cameras and would be patrolling the site at night.
Senior Constable Graeme McPherson said police wanted to hear from anyone who had seen or heard anything suspicious.
Mr Liu said the rooms in the motel complex were ready for guests and it was hoped to open next month, depending on contractors.
A restaurant and cafe would be added, possibly next year, and there were also plans for backpackers accommodation in the future.
He was very proud of what had been achieved.
