The Arson Squad and Busselton Detectives are investigating two separate bushfires in Yallingup and Injidup which occurred on Saturday night.
Both fires, which threatened lives and homes within an hour of being reported, appear deliberate and are being treated as suspicious.
South-West District Superintendent Geoff Stewart said the state arson squad would remain in the region to support detectives investigating the fires.
He said the situation could have been "scary" if it had not rained on Saturday evening.
"Unfortunately that area has been through things like that before and it is something no one would hope to go through again, thank you rain," he said.
"The rain certainly assisted first responders, thankfully no one was hurt.
"That is a godsend out of all this, all we do now is slip into the investigation phase. We will do everything we can to find out who may have been responsible for this."
It is the third bushfire in the same area that police are investigating as suspicious.
A fire in Injidup on December 7, 2020 is also under investigation, along with another fire which occurred in December 2019
Mr Stewart said police would investigate whether those fires were linked.
His message to the arsonist is, "stop it."
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"My message to the community if you have any information at all or if you were in the Injidup area or Yallingup town site area or Mount Duckworth area on Saturday afternoon or evening, if you saw something or someone that was suspicions or you consider to be suspicious please contact Crimestoppers," he said.
"If you have any dashcam or CCTV that could assist the investigation please contact Crimestoppers, we need your assistance with this investigation.
"We always get good support and we want to see that in this investigation."
Mr Stewart said people had already volunteered information and they need everyone else to do the same.
"If you have anything at all no matter how small it is, if you see a policeman out on patrol in the South West stop and tell them or report it to Crimestoppers."
Investigators are appealing for anyone with information regarding the fire to call Crime Stoppers on 1800 333 000 or to make a report online at crimestopperswa.com.au.
