The United Nations refugee agency is urging Australia and other countries to end the detention of child asylum seekers, saying it is against international law and breaches the rights of children.

This month marks 25 years since the UN Convention on the Rights of the Child was created, and Australia is a signatory. It is considered the most widely ratified human rights treaty.

But the United Nations High Commission for Refugees (UNHCR) representative in Indonesia, Thomas Vargas, said Australia's detention of asylum seeker children is a breach of it, and international law.

"We would urge any government, any government in the world, not to use children as pawns to solve a problem that is, by the way, not going to be solved by a unilateral approach," Mr Vargas said.

"Children do not belong in detention and it is clear under international law that states should not detain them."

He urged Australia to provide community-housing options for underage asylum seekers who arrived by boat.

Mr Vargas said Australia had more than 600 child asylum seekers in detention.

"The impact, the negative impact that it has on a child's life at the very beginning of their life, to put them in this type of horrible situation, you can just imagine the negative consequences that it has on their psyche, on their wellbeing," he said.

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"Not only is it not humanitarian, but it's illegal under international law."

Immigration Minister Scott Morrison said the Government was working to reduce the number of children in detention.

"There is now an almost 50 per cent reduction across all centres, including offshore processing centres," he said.

"We will be able to get the remaining children out of detention in Australia when legislation currently before the Parliament is passed."

New policy will not stop the boats: UNHCR representative

He also said Australia's new policy of limiting the refugee intake from Indonesia would not stop boats.

Mr Morrison last week announced Australia would limit its intake of refugees via the UN in Indonesia and refuse anyone who registered in Indonesia after June this year.

Indonesia's foreign ministry summoned the ambassador to let Australia know the announcement had "spoiled" the relationship.

Mr Vargas said people were still getting on boats and Australia's policies would not stop them.

"It's not going to solve the problem," he said.

"It may, in the short term, show some boats being stopped but these boats continue to leave from various places in the region and outside of the region.

"This policy, a unilateral policy from any government is not going to stop it."

He said 100 asylum seeker boats had departed towards Australia since the start of the year.

Last week an asylum seeker boat was found in Micronesia, from where the passengers and crew had hoped to reach Australia.

"Whatever the policy, we'd like to see legal options available," Mr Vargas said.

"If legal options are cut from under the table, people will then look to other ways of moving irregularly.

"This is why it's so important and this is why UNHCR continues to advocate for governments to come up with a regional approach."

Mr Vargas said countries needed to work together, not alone.