These companies (Billabong, Quicksilver) have their own set of authorized distributors in Australia, and the only way to compete with them is to become a distributor yourself OR to source from liquidators. Unfortunately, most of the liquidators are based in the USA, and I failed to locate any possible wholesale suppliers from countries near Australia.
These are Australia based clothing suppliers, but they do not carry the brands you have listed -
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Sorry that I wasn't able to provide much help. :(