rexcited is right on the money. Even if the property is big enough, if there are no vermin or reason for you to have a 308 you won't get one. And there is no set size either, the WA firearms branch specifically state this - its a "common sense" judgement

And you can't shoot roo's without a permit.

Minimum property size for calibre in WA - Shooting.com.au

Read that for some more info - WA has some of the tougher gun laws in australia, and they are only getting worse. goodluck