I assume that most country people and travellers know that there are superior reception range mobile phones, look for the blue tick identification mark.
And even not far away, several years ago I was on my boat with my son visiting friends who had a waterfront holiday home on the Hawkesbury River north of Sydney, my son tried to phone them to advise when we expected to arrive but his phone had no signal. He used my blue tick country phone with no problem.
There is no definitive answer to your question there is only a subjective one and that may change with each passing shower, phase of the moon or the density of the local trees.
Mobile phone boosters are generally only of any use to people in a fixed location (usually their home) who can also erect a good yagi antenna in the best spot and point it in the correct direction.
The last time I checked the only legal boosters were sold by the major service providers and A$1500 sticks in mind as a price. And (not directed at the OP) don't even *think* of using a non certified one from e-bay etc the ACMA *will* hunt you down and prosecute. Unqualified transmitters in the mobile bands are very high on their list of search and destroy devices and they do have some excellent direction finding gear - think multi station time-of-arrival coupled receivers - can't hide from that.
There's not much which can be done to improve the mobile coverage situation (else I'd have done it). It's almost (is?) impossible to find a phone with an external antenna socket and although I have a USB modem with such and a 13 element yagi it really doesn't help with data speed worth a damn in marginal coverage areas.
"Blue Tick"? Yeah, maybe, perhaps; the last I heard was if the bloke in the test labs who did the Blue Tick certification was off work then no certification could be done because to use another individual to hold the phone during testing changed the antenna impedance such that the tests were not repeatable... which is what I would expect. RF at 1GHz and above has a deal of black magic associated with it.
Essentially: if you want to go serious bush in Australia forget mobile phone - HF radio works anywhere but has very low bandwidth, satellite phones work most places, have variable but decent bandwidth (although rotten latency) and will cost you a fortune to run. I use Amateur Radio HF for voice comms and e-mail and leave web surfing until I'm in mobile range.
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As post above, the only certified legal booster is the Cel-fi go. These can be had for around $900, cheapest ive found is $845 on the net. 12v powered unit with external antenna to recieve signal and small flat antenna for inside of vehicle. This basically turns your vehicle into a hotspot. You must however have a signal to start with for the unit to work. Generally 1 bar will be boosted to nearly full if not full signal. I have been looking for sometime but just havent pulled the trigger due to cost
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