when you get the email back confirming pre-approval you then send your passport and paperwork and payment plus a return prepaid postage bag to the Sydney consulate
If you staying airside and not going through immigration at Bkk airport you dont need a visa as your not entering the country, different story if you have to go through immigration and check into your next flight
Do you have a motor bike license at home ? If not you will not have any insurance. Moped are not in thailand they are bigger than 50cc so need a license