There's been plenty of posts on this forum about clearance, have a look back over the past year.
Dealing with Customs and Quarantine is the easiest part of the whole exercise - the hardest being dealing with the koalas for the import approval.
If you have a day to spare you can do the clearance yourself and save about $500 to $1000 in agents/transport fees.
Basic process is;
You will receive the bill of lading from the shipping (as in the ship) company (your title to the goods). You go to the shipping company and pay their port fees and get a release order and you MUST get the ship's name, its voyage number and its first port of call in Australia so that you can complete the customs forms.
You then goto Customs, fill out a "Nature Ten - N10" form, pay any fees, GST or duty (if applicable), book and pay for the Quarantine inspection at the same time. Customs officers will help you complete the correct tarriff classification.
Quarantine and maybe Customs will give you a time and place for inspection and you must be present if you are doing all this yourself.
If the bikes pass quarantine inspection they are yours to take away after you have paid the stevedores (you may have to pay for crate disposal) - see other threads about what you can do at this time. vehicles brought into Aus via the import approval scheme are not roadworthy until they have the personal import approval inspection - whereas on a carnet you can drive the vehicles off the wharf (mad eh?). You need to organise a vehicle engineer to do the inspection, then get a roadworthy certificate/safety cert, then pay for registration. It varies slightly depending on the State you register the bikes in.
If they dont pass quarantine this is where it can get expensive as you will have to get the bikes transported to an approved cleaning facility, back to the inspection and then pay for another inspection and the clean. If you goto the Quarantine website they give cleaning standards as "new", make sure the crate meets standards as well. The inspection is a bit variable but assume that any vehicle thats been to the UK is assumed to be carrying foot and mouth and will get a good looking at - seeds and soil will get you a knockback. Its best to get inspections done on the wharves as its usually just a case of taking the crated bikes to the washer on a forklift - whereas agents tend to like to get the crates off the wharf to another warehouse.