Australia is a coverage question before it is a price question
Coverage follows the coast and the population. Sydney, Melbourne, Brisbane, Perth and the corridors between them are well served. The interior is not, and the difference is not marginal.
If your trip includes the outback, long highway drives or the less visited parts of Western Australia and the Northern Territory, plan for extended periods with no signal from any operator.
New Zealand
Better proportionally than Australia because the country is smaller, but the same pattern applies. Cities and main highways are covered, the fiordland and alpine areas are not.
Hiking routes in particular should be planned as fully offline. Download the maps and tell someone your route.
The Pacific islands
Fiji, Samoa, Tonga and the smaller island nations have limited networks and limited roaming agreements. A plan that lists Oceania may or may not include them in any meaningful way.
If your itinerary includes the islands rather than just Australia and New Zealand, check them individually. This is the part of the region where headline coverage claims are least reliable.
How to decide
Australia and New Zealand together is the common itinerary and the case where a regional plan is straightforwardly useful, because the two countries do not share networks.
Either country alone is a country plan. And in both, treat the coverage gaps as a planning input rather than something to be solved by buying differently.
Where are you going?
DestinationsCommon questions
Will I have signal in the Australian outback?
Usually not. The gaps are geographic and apply to every operator. Plan those legs offline.
Does one plan cover both Australia and New Zealand?
A regional plan is designed to. The two countries have separate networks, so a single country plan will not follow you across.
Are the Pacific islands included?
Often only nominally. Check Fiji, Samoa and Tonga individually if they are on your route.
Is local prepaid a better deal?
In Australia and New Zealand it is competitive and easy to buy. The trade off is the same as everywhere: price against arriving connected.