Can't comment on Perth as we are in Brisbane but we know of a few different agencies in Australia who work with Shopify Plus customers. It really depends on what work you are looking for as any Shopify developer/agency can probably assist you - no need to be listed as a plus partner to work with plus.
Bundleup - bundle your products and unbundle your orders. Channelup is a product data management solution helping Shopify stores automate their processes. If you require custom development or would like to test out our features contact me at firstname.lastname@example.org