Please follow our below definitions:
An off-market property is a property that is for sale that is not officially marketed to the public. Hence you will not find them advertised for sale on any real estate portal like domain.com.au, realestate.com.au, any Google or Bing search results or even real estate agent websites.
A pre-market property is a property that is for sale but not yet marketed to the public (but will be soon anywhere from a couple of days to a few weeks). Eventually it will be listed for sale on the real estate portals like domain.com.au, realestate.com.au and real estate agent websites.