How did 27 asylum seekers arrive on Christmas Is undetected?

HOW 27 asylum seekers came to arrive on Christmas Island, survive without detection for days - and embarrass the Federal Government at the same time - will now be scrutinised as Immigration Minister Scott Morrison aims to avoid it happening again.

During his weekly briefing today, Mr Morrison said while the inability to discover this wayward boat before its arrival "raises a number of questions", he said it was still "a very big ocean" to patrol.

The boat-load of asylum seekers landed either late Monday or early Tuesday on a remote part of Christmas Island.

When some of those on board were discovered on Thursday, the Australian Maritime Safety Authority launched a land and sea search operation.

All of those on the SIEV - or suspected irregular entry vessel - were found, with 22 of them now being kept at the Phosphate Hill facility on the island.

An adult man remains in hospital in a stable condition and four have been transferred into custody.

Before their discovery, it is believed they subsisted on coconut and crab before some ventured to a main road for help.

Mr Morrison said there would be a "post-operational assessment" so the department could learn from the incident.

"These 27 people who are now safe should count themselves extremely fortunate that they did not suffer a worse fate," Mr Morrison said.

"Anyone getting on a boat should not think there is an Australian safety net waiting for them."

Mr Morrison said he expected an increase in boat arrivals as people smugglers attempted to capitalise ahead of the treacherous monsoon season.

In the past week, 162 asylum seekers, six crew and 27 "undetermined" people arrived in Australian waters aboard four vessels.

Earlier this week, Mr Morrison acted to ensure no asylum seeker awaiting processing could be given a permanent protection visa.