Three boats have arrived near Christmas Island since Sunday carrying more than 250 asylum seekers.

Yesterday a Senate estimates hearing was told the number of asylum seekers had fallen, but that the number expected to languish in indefinite detention was rising.

In 2011, 11,510 asylum seekers arrived in Australia by plane and by boat, compared with 12,640 in 2010. The United Nations High Commissioner for Refugees says the drop is because of the fall in the number of people arriving by boat.

But Immigration Detention Services Group deputy secretary John Moorhouse told the hearing there were now 51 refugees in detention - and facing indefinite detention - who had been given negative security assessments by ASIO.

Among them is Ranjini, a pregnant Melbourne woman taken to Villawood with her two sons on May 10, after having lived as a refugee in the community.

In total, six children are detained because their parents have been deemed to pose risks to national security.

Mr Moorhouse told the hearing the process of detaining people with adverse ASIO assessments indefinitely was Department of Immigration and Citizenship ''procedure'', rather than a requirement under law. But most parents preferred to be with their children, he said.

''The options that are available to us in these circumstances are extremely limited … the circumstances of the children is very difficult for everyone.''

The two-day hearing, due to continue today, comes as three boats carrying asylum seekers arrived over the weekend. One boat, which arrived on Sunday morning, carried an estimated 175 people, while the others, which arrived on Sunday night, carried 44 and 38 people. All were taken to Christmas Island.

The government says 37 boats with asylum seekers have arrived since the opposition refused to sign up to the Malaysia solution, blocked by the High Court last year.

Senator Kate Lundy told the hearing that the opposition should support the government's offshore processing plans.

''We would like to see the Coalition do the right thing in the public interest and support our legislation to permit offshore processing and the Malaysia arrangement.''

Meanwhile, it has emerged that 144 more asylum seekers have lodged court injunctions to prevent them being forcibly removed from Australia. They have joining four others who have lodged injunctions against their removal.

In November, Afghan man Ismail Mirza Jan became the first to fight his deportation in a court, winning a temporary injunction in the Federal Magistrates Court in Sydney. That case, and three others, are now before the High Court.

Five people have voluntarily returned to Afghanistan under an agreement between the two countries.

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