China paid the company nearly $US50 million between 2008 and 2011 so it could keep a satellite in Tonga's orbital space.
Prime Minister 'Akilisi Pohiva and the Public Service Association are suing Tongasat, claiming the money should have gone to the government.
The company is now appealing an August Supreme Court ruling that the payments were unlawful.
Lord Sevele, who was Tonga's prime minister at the time both payments were made, said the court ignored key evidence that was submitted.
