can i use a GPS only apple watch SE bought in Japan in australia?

Yes, it should work fine because the GPS-only model doesn't have to be compatible with the local cellular service providers!

M1 Mac mini, macOS 13.5.2;  Watch, series 7 GPS/cellular, watchOS 10.0.1; iPad Pro 2nd Gen, iPadOS 17.0.1; iPhone 12 Pro, iOS 17.0.1; TV 4th Gen, tvOS 17; HomePods, audioOS 17 and Airport Express w/ Airplay 2

Just be aware that the warranty is for Japan, and may not be honored in Australia.