The End Immigration Detention of Children campaign is a coalition of non-government organisations from around the world, with the backing of the United Nations Human Rights Council.
Australia is the first country the campaign is singling out for attention after its launch in March.
A campaign spokesperson says the youngest unaccompanied child in Australian detention centres will have spent a year behind bars this month.
She was six when she arrived on a boat without her parents.
Early Childhood Australia says there are 428 children awaiting processing in immigration centres around the country.
"The government needs to show moral leadership in dealing with the complexities of this situation," chief executive Pam Cahir said in a statement on Tuesday.
Her group, which acts as an advocate for young children, wants the government to place families with children and unaccompanied minors in the community while their claims are assessed.
She says the trauma children experience from loss, being uprooted from their home and culture and other damaging experiences is made worse by long stays in detention.
Children who have spent time in detention centres will meet politicians in May.
A video petition, titled Speak up Behind Bars, is aiming to get at least 428 messages of support for detained children - the same number as there are children in the centres.
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