Mr Saunders said the one positive was that camping gear inside the hall had not been damaged by the fire.
The gear was essential for Easter camp, which members would be going on in two weeks.
Mr Saunders said the hall could be repaired, according to an initial insurance assessment, but it would be out of action for some time.
"The back wall was damaged, the leaders' room was basically gutted. The kitchen is gutted and another smaller room has been gutted."
Mr Saunders said Scout leaders would meet tonight to decide how to accommodate the groups who would be displaced by the fire.
Detective Sergeant Mike Priston of Wellington police said the offender or offenders went underneath the hall and set fire to part of the floor about 3.30am.
"If anyone has any information about the suspicious fire I would encourage them to come forward," he said.