This is one of the most popular questions, as we learned from VPC Very Popular Cars, and here is the answer from their website:

According to paragraph 17 of the Traffic Act of Tonga, any person with a valid driving license issued by another country that recognizes Tongan driving licenses may drive in Tonga for up to 12 months. This is allowed as long as the individual holds a license that permits them to drive the same class of motor vehicle under the same conditions as their foreign license. The reciprocal recognition of driver's licenses applies to countries that have a visa-free entry to Tonga.
  • Australia
  • Austria
  • Bahamas
  • Barbados
  • Belgium
  • Brazil
  • Brunei Darussalam
  • Bulgaria
  • Canada
  • China (mainland)
  • Cook Islands (New Zealand passport)
  • Cyprus
  • Czech Republic
  • Denmark
  • Dominica
  • Estonia
  • Federated States of Micronesia
  • Fiji
  • Finland
  • France
  • Germany
  • Greece
  • Hong Kong
  • Hungary
  • Ireland
  • Israel
  • Italy
  • Japan
  • Kiribati
  • Latvia
  • Lithuania
  • Luxembourg
  • Macao
  • Malaysia
  • Malta
  • Marshall Island
  • Monaco
  • Nauru
  • Netherlands
  • New Caledonia (French passport)
  • New Zealand
  • Niue (New Zealand passport)
  • Norway
  • Palau
  • Papua New Guinea
  • Poland
  • Portugal
  • Romania
  • Russian Federation
  • Samoa
  • Seychelles
  • Singapore
  • Slovakia
  • Slovenia
  • Solomon Island
  • South Korea (Republic of Korea)
  • Spain
  • St Kitts & Nevis
  • St Lucia
  • St Vincent & The Grenadines
  • Sweden
  • Switzerland
  • Tahiti (French passport)
  • Tokelau (New Zealand passports)
  • Turkey
  • Tuvalu
  • Ukraine
  • United Kingdom
  • United States of America
  • Vanuatu
  • Wallis & Futuna (French passport)
According to paragraph 18 of the Traffic Act of Tonga, any person holding a valid driving license issued by a country that does not recognize a valid Tongan driving license, due to lack of reciprocity, can apply to the Principal Licensing Authority. To do this, they must present their foreign driving license, meet the necessary requirements for the recognition of that license, and pay the prescribed fee. If all conditions are satisfied, the Principal Licensing Authority may issue a Tongan driver's license that allows the individual to drive the same class of motor vehicle as their foreign license permits in Tonga.  

Please ensure you are ready to comply with the Traffic Act of Tonga before your arrival.

We wish you safe driving in Tonga!