An independent UN investigator into the human rights of migrants has postponed an official visit to Australia that was due to begin today, citing a lack of government cooperation and "unacceptable" legal restrictions.

Asylum seekers have long been a lightning-rod political issue in Australia. UN Special Rapporteur Francois Crépeau had planned to gather first-hand information about the situation of migrants and asylum seekers in the country and in Australian off-shore detention centres in neighbouring Nauru and Papua New Guinea. He had asked for access since March but Australia denied him access to any offshore centre, he said in a statement.