What would the conditions they impose be. Does anyone from customs actually turn up at the aircraft and stamp your passports as you board.
Correct, although outbound Aussie passports are no longer routinely stamped (no kidding, it saved a fortune in ink). There used to be a charge for their transport from the nearest manned Customs port, although I don't know if that's still the case. That may be the fee mentioned. There is also a limit on the number of pax you can have, but from memory it varies from office to office (it was max 15 POB here, any more and you had to go to an international airport).
If you are departing there is no need to worry about Quarantine but if you are an arrival you need to contact AQIS in advance so they can send out an Inspector as well to physically clear your aircraft. Possibly in more far flung places they deputise Customs, but you would need to check with them.
On arrival the clearance process should be reasonably straight forward, but be aware that Customs are fully entitled to turn up with a full search team, mobile X ray and car full of drug dogs if they feel like it. 9 times out of 10 Customs will just want your paperwork and a quick peek inside the aircraft, and AQIS will want evidence of you bug spraying it and check it for fruit etc.