The statement came as the incoming president of the Law Council of Australia, Fiona McLeod, SC, said the continued detention of children could place Victoria in breach of international conventions to which Australia is a signatory.

On Thursday, Ms Mikakos provided written responses to Greens children and families spokeswoman Nina Springle on the conditions in which children were being held.

Previously, the minister and her department had said only that "standard management" of young people in detention did not involve 20-hour lockdowns.

But Ms Mikakos' answers raise further questions about the conditions in which children are being kept at the Grevillea unit at Barwon – which was hastily repurposed into a youth justice facility three weeks ago – on the eve of a Supreme Court challenge against their detention.

"The use of lockdowns and isolations are operational decisions made by my department," Ms Mikakos said.

"It is sometimes necessary for young people to be isolated or for youth justice units to be locked down due to imminent security or safety risks to themselves, others or the security of the centre."

She did not say which of these was the reason children had been kept in their cells in lockdown, but lawyers have told Fairfax Media that all children being held at Barwon are almost continually held in their individual cells.

In her response, Ms Mikakos also confirmed that "separation" has been used as a way to control children's behaviour.

"Where a young person displays consistent or extreme violence or destructive behaviour that has continued despite attempts to prevent it, their separation may be appropriate and a management plan is put in place," she said.

Ms McLeod said: "What we've done here is go from case management to solitary confinement in an adult jail.

"You can't tell me there is not a more appropriate and proportionate response available, to put them in these sorts of conditions rather than provide them with support."

The public intervention into a state matter by the Law Council, which represents 65,000 lawyers across the country, is a highly unusual move.

Ms McLeod said she was spurred to speak out by her concerns Victoria was in breach of at least two international conventions – the Convention Against Torture and Other Cruel, Inhuman or Degrading Treatment or Punishment, and the Convention on the Rights of the Child – by detaining children at Barwon.

On Monday, the Human Rights Law Centre and Fitzroy Legal Centre will return to the Supreme Court to challenge the legality of children being held in adult jails.

They will argue that the government has acted unlawfully and is in breach of the Victorian Human Rights Charter.

The government is defending the action.