The federal government has accounted for the whole of a group of asylum seekers who arrived undetected on Christmas Island.

Immigration Minister Scott Morrison said there were 27 people in the care of his department.

The tally included the nine people who were unaccounted for earlier on Friday.

The group arrived on the southeast coast of Christmas Island on Monday evening and had been living in heavy jungle around Greta beach.

"This has been an unusual incident that raises a number of questions that will be subject of a standard post-incident operations assessment," Mr Morrison told reporters in Sydney.

To see all developments regarding Operation Sovereign Borders follow the SBS story stream.

Mr Morrison defended the inability of Australian authorities to intercept the vessel.

"This is a very big ocean," he said.

"These are very small vessels and these 27 people who are now safe should count themselves extremely fortunate that they did not suffer a far worse fate."

The minister said the incident should serve as a wider warning to people thinking on getting on boats to Australia.

They should not think there is an "Australian safety net" waiting for them.

"I am sure that all Australians will be very pleased there has been no loss of life on this occasions" Mr Morrison said.

Lieutenant General Angus Campbell said it's believed the people came ashore at Greta beach on Monday evening.

The fate of their vessel has not yet been confirmed but it's believed to have sunk, he said.

"These people are extremely lucky to have survived their ordeal," Lt Gen Campbell said.

"They had a very difficult and distressing time," he said.

National parks and wildlife officers on Christmas Island informed Customs at about 2.40pm AEDT on Thursday that they had discovered the group, he said.

He said, while undetected arrivals have occurred in the past, he has ordered the commander or Border Protection Command to conduct a review to mitigate against future such "surprises".

In the past week, there were three boats intercepted, carrying a total of 162 asylum seekers and six crew.

The group of 27 were found on Christmas Island after they came ashore at a remote part of the island.

Of those, one was in hospital receiving treatment for minor injuries.

Mr Morrison said there would be clusters of boats trying to reach Australia ahead of the monsoon season.

Lt Gen Campbell said there could be a number of reasons for a cluster of four boat arrivals this week, including the weather and the ability of people smugglers to bring together prospective travellers.

"We are not in a position to give you a confident statement of why we have a cluster today," he said.

Asked if it was linked to Indonesia halting co-operation on people smuggling in the wake of revelations of phone tapping by Australia, Mr Morrison said it was not the first time there had been a cluster of boats in a week.

"I don't think this needs to be over analysed," Mr Morrison said.

He said Australia and Indonesia were repairing their relationship.

"What is important is that the foreign minister (Julie Bishop) was in Jakarta yesterday (and) there is a process that has been agreed between the leaders that is being worked through," he said.

Mr Morrison said there was no one partner or no one measure that would bring the end of people smuggling.

