THE remote Cocos Islands are being freshly targeted as a destination for asylum seekers, creating a nightmare for Australian border surveillance authorities to cover thousands of square kilometres of open waters.

Three boats carrying 135 asylum seekers have arrived since May 16 at Cocos Islands, an Australian territory in the Indian Ocean just over halfway between Australia and Sri Lanka.

Sri Lanka's top envoy in Canberra has also confirmed his government recently stopped a boat carrying more than 110 people departing for Australia via the Indian Ocean.

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People-smuggling syndicates have not historically targeted Cocos Islands, preferring to send boats to Australian territory closer to Indonesia - either Christmas Island, south of Java, or Ashmore Reef off West Timor.

The prospect of a third route for boat arrivals is bound to stretch Australia's border patrols, already struggling with a recent surge in smugglers' operations.

The distance between Ashmore Reef and the Cocos Islands is more than 4000 kilometres, a massive expanse to patrol.

Few boats have made the nearly 3000-kilometre direct journey from Sri Lanka to Cocos Island. One did in 2011 and another in 2010, which had to be rescued.

Opposition border protection spokesman Michael Keenan said most of Australia's patrol boats were acting as taxis to ferry people to Christmas Island, making it impossible to cover such an enormous zone.

More than 4000 people have risked a boat journey to Australia so far this year - almost the same number as for all of last year. Another two boats carrying 150 people were intercepted off Christmas Island yesterday.

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Refugees' new route to Australia