By Colin Packham

SYDNEY (Reuters) - A U.N. investigator into the human rights of migrants arrived in Australia on Tuesday to assess its tough asylum seeker policy, after aborting a visit last year claiming a lack of government assistance and access to asylum seeker detention camps.

The U.N. Special Rapporteur on the human rights of migrants, Francois Crépeau, will spend 18 days in Australia and the tiny South Pacific island nation of Nauru - where some 410 men, women and children remain in indefinite detention.

"This is an opportunity for me to understand how Australia manages its overall migration policies, and their impact on the human rights of migrants," Crépeau said in a statement.

Under Australia's border security laws, asylum seekers intercepted trying to reach the country by boat are sent for processing to detention camps on Papua New Guinea's Manus island and Nauru. They will never be resettled in Australia.

Currently, there are 1,200 people held in camps.

A U.N. report in October heavily criticized Nauru for its failure to protect asylum seeker children from sexual abuse inside the Australian-funded detention center.

Crépeau's visit comes amid fresh criticism of Australian Prime Minister Malcolm Turnbull over a government proposal to permanently ban asylum seekers who arrive by boat from ever entering Australia again. Even if they are resettled in a third country as genuine refugees they will be denied any Australian visas in the future, even tourist or business visas.

"We welcome refugees, they have made an enormous contribution to Australia...but we are able to do that because we can maintain the integrity and security of our borders," Turnbull told Australian Broadcasting Corporation radio.

Crépeau, who canceled his last visit citing a lack of government cooperation and "unacceptable" legal restrictions denying him entry to Nauru, will issue his findings in a report on November 18.

"It's clear Nauru and Manus are dead ends. The government urgently needs to find a humane way forward," said Daniel Webb, director of legal advocacy, Human Rights Law Centre.

(Reporting by Colin Packham; Editing by Michael Perry)