Tonga's Heritage Society is starting legal action in a bid to stop the government from developing what it calls a site of huge historical and cultural significance.
Preparation has begun in Popua to build a golf course, a park and canals.
Society president 'Ana Taufe'ulungaki said the site was where Tongan royalty was established, where the Lapita people arrived, and where Captain James Cook and his crew were welcomed.
Dr Taufe'ulungaki also said the area hosted ancient royal pigeon snaring grounds and played a large part in Tongan mythology.
In 2015, Parliament accepted a petition and agreed to protect the site but Prime Minister 'Akilisi Pohiva is now leading developments there.
Dr Taufe'ulungaki said her group would be seeking a judicial injunction to stop the work.
"That's the only option that is now available to us. We didn't want to go through this route but we are forced to do so by a government who is unwilling to negotiate or to listen, or to follow the rule of law," she said.
Dr Taufe'ulungaki expected to have their action filed by the end of next week.