Cash is king in Tonga (dig the new plastc notes to celebrate King Tupou VI's coronation): be sure to take plenty with you to Ha’apai and ‘Eua which are ATM-free. There are ATMs, however, in Tongatapu and Vava’u, and it’s easy to change major currencies at local banks.
Credit cards are accepted at many tourist facilities but often attract a 4% to 5% transactions fee. Visa and MasterCard are the most common.
Tongans don’t expect tips but you won’t cause offence by rewarding good service with a few pa’anga.