Western Australia is separated from farms in the rest of the country by ~1000 km of desert, meaning there are various pests in the rest of the country that they don't want to see introduced, and thus enforce strict quarantine measures: https://www.agric.wa.gov.au/importing-animals/quarantine-wa-...
FWIW, California also has agricultural quarantine measures, they're just much more half-assed.
Hawaii does not play around with its own agricultural (et al) quarantines. Every time you fly into the state, you must declare items on a special state checklist, for exactly the same reasons.
Australia is an island so it has a fairly unique biodiversity. For international flights bringing in fruits is a concern since it can introduce pests that don't otherwise exist in Australia.
Australia is also a very large island, and the biodiversity of the east coast is very different to that of the west coast, so the same concerns apply. If you drive along the highways between states you'll see signs telling you to dispose of your fruit.
We take protecting our agriculture very seriously. You don't want an outbreak of something in one state to spread to another.
It kinda makes sense. We're so isolated geographically, when something happens in other parts of the world, we have a very good chance of protecting ourselves.
I believe Australia actually has the oldest vineyards in the world. Something took out the vineyards everywhere else in the world.
Diseases in one part of the country could spread to another. Australia and NZ have strict biosecurity because their isolation means they missed out on a lot of diseases that affect produce and animals in other parts of the world.
Because there are crop diseases endemic in one part of the country that aren't present in another. Try searching for "Nevada California fruit" if that's closer to home.
