King Taufa'ahau was the uncle of the dead prince
Two members of Tonga's royal family have been killed in a car crash during a visit to the United States.
Prince Tu'ipelehake, Princess Kaimana and their driver died when a teenager crashed her car into their vehicle.
The two royals were visiting members of the Tongan community in the San Francisco Bay Area to discuss political reforms in the Pacific archipelago.
Prince Tu'ipelehake, a nephew of King Taufa'ahau Tupou IV, was the leading reformist in the Tongan royal family.
Tonga is the last monarchy in the Pacific and most of its politicians are appointed by the 88-year-old king rather than elected.
New Zealand Foreign Minister Winston Peters described the deaths as a "tragedy" and praised Prince Tu'ipelehake's efforts at reform.
"He pursued this goal with sensitivity and perception, mixed with a strong determination to achieve progress," Mr Peters said.
The accident happened on Wednesday in Menlo Park, about 50 kilometres (30 miles) south of San Francisco.
Police said Edith Delgado, 18, was racing her car on the highway when she tried to overtake the sports utility vehicle carrying the royal couple.
Ms Delgado's car slammed into the driver's side of the SUV, causing it to swerve across several lanes before tumbling to a stop on its roof.
Prince Tu'ipelehake, 56, Princess Kaimana, 46, and their driver, Vinisia Hefa, 36, were all killed.
Ms Delgado, who was uninjured, has been detained on suspicion of manslaughter and engaging in a speed contest, police said.
Tonga is a 170-island archipelago about halfway between Australia and Tahiti.
It has a population of 108,000 and its economy is dependent on pumpkin and vanilla exports, fishing, foreign aid and remittances from Tongans living abroad.
Prince Tu'ipelehake had been scheduled to speak to Tongans in San Bruno, California on Thursday before visiting Tongan communities in other parts of the US.