The Vanuatu opposition leader, Moana Carcasses, says the government's deportation of an Australian-based lawyer, Robert Herd, was illegal.
Mr Herd was flown out of the country on Thursday amid reports that he had had political links.
The prime minister, Joe Natuman, has recently warned foreigners not to become involved in politics.
But Mr Carcasses, who is the country's previous prime minister, says the government's actions send an appalling message to the international investment community.
He says the section in the Immigration Act used can only be applied if there is a threatened breach of national security, defence or public order.
Mr Carcasses says it shows Mr Natuman is determined to throw out the reforms his government had made to combat corruption and illegal practices.