I have purchased almost all the camera gear I own from O/S because of the rorting distributors/retailers here in Australia. I have been hit several times by Customs upon importing gear, and you need to consider that when you are buying O/S.
Some places like the big USA retailers, B&H, Adorama, KEH etc use couriers and will not "modify" the item value on the customs declaration. Because they use UPS, FEDEX etc, those courier companies (for their own benefit) at the time of collecting the item from the seller O/Sflag any item with a declared value above the threshold of the country that the item is being imported into for collection of duty/GST. Because these couriers are approved by Aus Customs to clear goods in house, they just love to clear your goods by then collecting the relevant duties/GST AND adding their own hefty (often $150+) customs broker charges, so they will almost never let anything where duty would be payable slip through, and sometimes their fee can be more than the duty/GST you are liable for.
Asian sellers from HK, Korea, SG, TW, CN etc will often be far more helpful regarding the declaration of item value.
If I bring anything of real value in, I will only work with sellers who will help me regarding duty/GST and I only ever allow the use of UPS/FEDEX if the shipment is entitled to pass free of duty/GST, so ie: has a total value including freight of <AUD$1K.
When sellers use USPS, items that are only marginally above the AUD$1K limit will often come in without being picked up by AUS Post, but it is rare that an item declared at 2-3 times the limit will go unnoticed. One aspect that is very useful (esp for camera gear) is that items that are being imported as a warranty replacement or being returned to the Australian owner following a warranty repair are entitled to pass free of any duty/GST regardless of the item value. This was very helpful for my "Warranty replacement" of the Nikkor 200-400VR that I recently purchased. If you are buying from an individual, they are often helpful regarding declarations as well.
I will do my own customs clearances if anything I bring in requires it, the cost is about $43.50 last check, plus any duty/GST that applies. You need to register with Customs first time and it can be dine via fax or email, just a couple of forms and some ID. The first time it will take 4-7 days to release your item if you pay at the first opportunity, but once you have an importer ID, it takes 1-2 days to get things released once you pay. Customs brokers and approved couriers can clear things immediately once you pay, but they will often charge 4-5 times the price of doing it yourself.
Just be sure to ask your seller about their declaration requirements, and importantly ask what shipping co they are going to use because it can really eat the savings you may make. Almost all still camera gear can be imported duty free, however items with a value that exceed the AUD$1K limit including freight are liable for 10% GST on the total AUD value. The AUD value will be determined at the exchange rate that was current on the day the declaration was made by the seller. Customs have all of this inf on their website in .PDF format including the exchange rates for each day so you can read in more detail there.
Hope that helps.