Interesting item and a datestamp that I certainly haven't seen before. I don't think it's strictly a postmark but a P.M.G.'s Department datestamp used at the Hobart GPO in the relevant department (i.e. Deputy Radio Inspector) to record outwards and probably inwards mail too.
Throughout the 1920s through to 1939, the Deputy Radio Inspector in Hobart was Mr. E.J.G. Bowden (I think he may of been ABC broadcaster Tim Bowden's grandfather but that requires further research)
If you would like to find out what the Deputy Radio Inspector actually did, there's a lot of reading ahead of you in this link!http://trove.nla.gov.au/newspaper/result?q=Deputy+Radio+Inspector