Biosecurity is a major concern in Australia. There are many diseases that we are free of that could wipe out native flora and fauna, or severely effect agriculture. And as has been previously stated there are also internal border security checks. There are areas within NSW where it is illegal to enter with fruit in an attempt to stop the spread of fruit fly and there are state border checks. On my first visit to South Australia I was asked at the border if I had any food and I declared some potatoes. They were confiscated. I asked for a couple back for my dinner as it was 6:00pm and stated I was staying in Renmark, just a few Kilometres away and would be cooking them. No chance. It is impossible to cross some state borders with any fresh food. When travelling I now cook any meals and freeze them as that is OK.
On my fishing trip to the Kimberley I had an old, empty 20kg birdseed bag which I was going to use to put my fish in. Because it said birdseed on the bag it was confiscated on the WA border. I guess a box that said oranges would also be confiscated.
And on my return from New Zealand I had purchased a vintage Stanley no55 plane complete with 4 boxes of cutters. Because the plane handle was timber and the boxes were timber I had to either forfeit them or have them treated at my cost. An extra $200 I had not planned on.
A friend imported some triple glazed timber windows from Finland. They were held up at the border and kept in safe storage until he could provide evidence of what the timber was and what forest it was cut from. Fortunately that eliminated the need for treatment of the timber, but he still received a bill for the time the windows were in storage awaiting proof.
Generally do not try and bring plant material, including timber or any fresh food or meat into Australia or do not cross any of our state borders with fresh food or plant material or it will be confiscated. And failure to declare can involve penalty infringement notices or legal proceedings.
But the real funny was the starling and sparrow patrol that protected the West Australian border with rifles from those imported pests, the English sparrow and the English starling. They might fly across the border into WA. These pest species are mostly concentrated in our cities and the vast Nullabour Plain across South Australia and central desert would surely be a natural barrier however the WA management plan for those species used to include armed patrols of the area near the border. Paranoia?