Under the Crimes Act (Vic) https://www.austlii.edu.au/cgi-bin/viewdoc/au/legis/vic/consol_act/ca195882/s87.html it mentions that you cannot menace someone in order to obtain some benefit or to cause someone a loss.
In this situation is it blackmail to threaten someone who has been evading taxes (Commonwealth of Australia) that you will make a report to the ATO about a relative of an individual if the individual doesn't help you investigate a matter over which THEY have authority and are not exercising it.