Police are appealing for information on Tuesday night's fires in Castlecliff.
The fires, which saw firefighters battling the flames for about four hours, are believed to have been deliberately lit, and travelled through the grass in the sand dunes at Morgan St.
Detective Senior Sergeant Neil Forlong said the case had been passed on to the Whanganui CIB to investigate further.
With the help of Rural Fire, police managed to get photos on Wednesday of damage from the blaze.
Two separate fires in the area joined to create one about 200m long, while a smaller fire behind the Duncan Pavilion in Rangiora St was put out quickly on the night.
"We're looking at a couple of possible people that were at the scene," Mr Forlong said.
"We would like anyone that was there, who may have information, to contact us."
-Anyone with information can contact the Whanganui Police Station on 06 349 0600.