According to the information contained in the "Am I In Business" page of the ATO website, there isn't a threshold as such, and each case is determined on an individual basis. However, when seeking information over the phone on the status of our business, we were given a figure of $50,000 as a rule of thumb on not one but three occasions. There were other conditions listed, but the general consensus was that anything more than that amount would be considered a declarable income. Perhaps it's something to do with the fact that at this level, charging GST is deemed to be mandatory rather than a choice by the business owner. Perhaps it's pure coincidence that those figures are the same. No idea, and I don't still have the names of these agents.