I do not know the regulations in Australia, where you say you are, but...
- in the USA I would have to talk to ATC and declare myself "pop up IFR traffic", they'll either accept me as I am or ask me to file a flightplan on RT, then proceed. I have the option later on to "Cancel Clearance", at which point I would revert to VFR. In the USA, for all reasonable purposes there is no class G above 1200ft, so this would be in class E.
- in the UK I would be entirely within my rights to fly semicircular rule, if I was in receipt of a service just tell them that I'm now IFR, and just carry on. If I go back to VFR (which isn't strictly necessary, I can happily continue IFR in VMC) I tell the service provider if I have one, and make a note for my logbook. That would indeed be class G.
As for your local regulations, no idea although filing a flight plan on RT is generally possible anywhere in the world, so you really shouldn't have to land to do that.
G