At one point in my job I was involved with taking trucks in a container between various parts of the world.
The trucks often went from Argentina / Indonesia into New Zealand or Australia.
We used to have jet was the vehicles for hours to try to get them clean enough. Sadly at one point we failed to meet the required level of cleanliness.
They had the trucks in a pit and ran their fingers along the chassis top with white cotton gloves on!
They rewashed the trucks and all the waste is collected then incinerated and you pay!
When your landing you will be given an imigration card. You are required to tell them if you work on or have visited a farm.
Jet washing the bike should be fine just be extra sure there is no dry mud anywhere, so yes under seat steerer tube as mentioned. They know where to look!
The thing they are worried about is soil / seeds entering their eco system. There is a big problem in Australia called die back where huge areas of plant life simply die as they have been exposed to bacteria never found there for thousands of years then imported unwittingly by travelers.
https://www.dwg.org.au/what-is-phytophthora-dieback