Immigration Minister Brendan O'Connor said on Wednesday the group would be sent to Christmas Island - despite having landed on the mainland - to deter others from making the same journey.

On Thursday he said that he had not yet received answers on why the asylum boat made it to the mainland, saying that people should not be too concerned about such a ''rare occurrence''.

Mr O'Connor told ABC radio that he did not believe that the boat's captains had adopted a new strategy to reach Australian shores, but he would need to get this confirmed by the agency.

''I think this vessel is . . . an exceptional case. We'll examine the reasons this has occurred. We'll certainly explain that publicly once we've confirmed that''.

The asylum seekers who were aboard the boat are now being held in custody and if determined not to be genuine refugees they ''will be sent home to Sri Lanka as almost 1000 have in the last few months''.

Meanwhile, Home Affairs Mr Clare said on Thursday that initial inquiries about why the boat took the route it did suggest it wanted to get to New Zealand, and not the Australian mainland.

Mr Clare echoed Mr O'Connor's comments that Sri Lankans who did not meet the requirements of the refugee convention would be sent back home.

Opposition defence spokesman David Johnston said on Thursday that a Coalition government would spend about $1.5 billion to buy and operate seven unmanned aircraft to patrol the Australian coastline.

The drone program would be fully operational by 2017-18, with the aircraft feeding information back to the navy and Customs.

''Obviously a hot spot like the northwest maritime frontier is a place where you'd put these things,'' he said.

Asylum seekers who arrive in Australian territories excised from the Migration Act - including Christmas Island - face being sent to offshore processing centres, and are unable to lodge visa applications while they are in Australia.

But Federal Parliament is yet to pass legislation that excises the mainland for migration purposes, in effect treating the mainland the same as excised territories.

Vince McMahon, head of the border security division in the Immigration Department under the Howard government, said history showed involuntary returns worked to reduce the flow of boats.

''It's extremely good tactics to return people as quickly as possible if they don't engage the Refugee Convention.''

Refugee lawyer David Manne said it was ''hard to see why it would be necessary'' to take the asylum seekers to Christmas Island for processing.

''They've come to the mainland, they have an entitlement to have their refugee case heard properly under due process,'' he said.

But he said under recent practices asylum seekers, particularly Sri Lankans, had been ''screened out'' of the refugee process before they had had the chance to have their claims for protection formally considered.

''It's extremely important here, and fundamental, they be able to access their basic legal rights and have a fair go before the law - and that means, first and foremost, they be able to have access to legal assistance.''

Opposition immigration spokesman Scott Morrison said the west coast of Australia was now effectively ''open game'' for people smugglers.

''What that means as we stand here today, is the entire Western Australian coast up to Darwin is all basically open game for people smugglers to land people in Australia and not have to have their passengers subjected to being transferred for offshore processing.''

But Mr McMahon said he believed the boat's arrival was a ''blip'', rather than evidence of an emerging trend.

''It all comes back to economics. The further you go the better the boat in general [needs to be] and the bigger the loss, vis-a-vis paying customers. Nothing has changed pre-Geraldton to now, other than we've had a boat arrival.''

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