Are the locations you're visiting in regional Victoria often the same?

^check proximity to the towers at the addresses you frequent most on the above map, go with provider that has highest ratio of close towers to places you often visit.

hint: 90% chance it'll be Telstra

The other helpful thing would be to get a 3G modem with an external antenna connector and get a high gain antenna - just for emergencies where the inbuilt antenna is struggling.
Bye now!