A small handful of countries have experience with preferential voting systems in Oceania (e.g. LPV). Maybe this is due to an Australian diffusion effect or even election assistance from Oz.

Tonga would add to a set of several current, governmental STV implementations (not including IRV implementations):

1. Malta – legislature
2. Ireland – legislature
3. Australia – upper house, several state and local assemblies
4. Northern Ireland – legislature, local assemblies
5. Scotland – local assemblies
6. New Zealand – some local assemblies
7. United States – some local assemblies (Cambridge, MA and, pretty soon, Minneapolis, MN)
8. European Union – some countries’ EU Parliament delegations
9. India – upper house (indirect)

And that list does not include historical implementations (e.g. briefly in Estonia, for decades in at least 2 dozen US cities, Nepal’s upper house before the republic).

If you don’t know how STV works, watch this video, focusing on the determination of threshold and the count. (Then, if you want to know more, watch a more specific video like this one.)