The deadly clashes on Manus Island last week flared after asylum seekers realised the Australian government was ''lying to them'' about plans to resettle them in Papua New Guinea, a migration agent who worked on the island has said.

Liz Thompson, who was helping asylum seekers lodge their refugee applications on Manus Island when the alleged riot broke out, told Fairfax Media that the asylum seekers launched their protests when a Papua New Guinean official went ''off script'' and acknowledged there were no plans in place for their resettlement.

Her claims came as Australia's top immigration bureaucrat told a Senate hearing that the post-mortem on the body of 23-year-old Reza Barati, who was killed in last Monday's clashes, still had not been carried out. Martin Bowles, secretary of the Immigration Department, also said he could not guarantee that local security contractors who had inflicted injuries on detainees had been stood down.