Not inevitable at all. Try arriving in Australia with a piece of fruit in your hand luggage, or try arriving in New Zealand with mud on your walking boots, and see what happens.
The seeds arrived months later, well after planting season, from Kazakhstan. The customs declaration called them plastic beads, but they were definitely seeds. I destroyed them, because I couldn't know for sure they really were what they said they were - seems unlikely a niche regional American plant would be available in Kazakhstan.
Would Australia really be able to stop these cases where they lie on the customs form?