As said you can be charged GST on any goods under $1k now. Over $1k there is usually extra customs and duties as per above.
If a company is doing a lot of business with australian residents the gov usually puts the pressure on for them to register for GST and charge it in their invoice. It happened with rock auto, they got pressure put on them and stopped selling to australians until they registered for GST.
Some of the smaller businesses aren't registered and you normally get away with not paying GST, but that was 9 months ago, maybe customs are pulling them up now. Avoid DHL and try for UK post, USPS or UPS for smaller items and there's never been any extra charges for me from certain smaller non GST registered businesses (I usually buy about $400 or so worth of BMW parts per year from overseas from pelican or fcp euro and have never been charged GST because they aren't GST registered and don't do much business in Australia). E: pretty sure FCP don't have invoices or declarations on the box either, just con/picking slip and sent via UPS.
But
OP is going to be slugged a lot more than GST, there's going to be freight and customs on ,so factor that into your costs.