Long-awaited reforms to youth justice take effect on Monday, meaning 17-year-olds charged from now to be dealt with in the youth justice system.

But new Child Safety Minister Di Farmer revealed capacity constraints meant the process of moving teens already in adult prisons to youth detention centres would be delayed.

"Modelling that we've seen to date indicated that we did have sufficient capacity in the youth detention centres, however, I do not feel confident that this is now the case," she said.

"Seventeen-year-olds currently in prison will stay in the adult prison until the director-general of my department determines that it is appropriate to do so and safe to do so."

Former police commissioner Bob Atkinson will oversee the final implementation of the government's youth justice reforms.

"My sense of it already is that this is a very wise and sensible course of action," he said.

Ms Farmer could not say how long the move would be delayed.

"What I've asked Mr Atkinson to do is to look at the processes we have in place at the moment to see if they are robust and they have integrity and Mr Atkinson is going to advise me on when is the right time to begin transferring eligible young people over to the youth detention centres," she said.

"It will take however long it takes to get this right."

Asked whether she knew how much extra capacity was needed in youth detention centres, Ms Farmer said: "That's why I've asked Mr Atkinson to have a look at [it]."

"I want to be confident about numbers and when he advises me, I will make decisions about the transfer of the young people to the detention centres."

There are 61 17-year-olds in adult prisons this week, who are on remand or serving sentences.

Of those, 35 would be potentially eligible to transfer to youth detention.

There were 48 17-year-olds in the state's adult prisons when the Palaszczuk government announced the legislative changes in September 2016.

The move was a 2015 election promise from Labor and came in the wake of footage that emerged of a 17-year-old detainee being placed in a helmet.

The LNP has for months questioned whether there was sufficient capacity in Queensland's youth detention centres to accommodate the additional 17-year-olds.

Ms Farmer said the government had already delivered upgrades at youth detention centres, trained 98 additional staff.

Mr Atkinson will also advise on recommendations related to youth detention stemming from the Royal Commission into Institutional Responses to Child Abuse.

The 2017-18 budget included $199.6 million over four years to transition 17-year-olds from Queensland's adult justice system into the youth justice system.

Queensland was the only state in Australia to hold 17-year-olds in adult prisons.

Community Legal Centres Queensland director James Farrell said children did not belong in adult prisons, and described the "11th hour announcement" of the delay as disappointing.

"It's unsafe, and goes against everything we know about ‘what works’ to address youth offending," he said.

"Queensland and Australia face regular international criticism for this regressive practice, and urgent change is required.

"We’ve had decades to get this right, and had hoped that changes would come into force today.

"Sadly, today’s announcement means that children will be left to languish in adult prisons again tonight."

LNP leader Deb Frecklington said the transfer of 17-year-olds was supposed to happen in November last year.

"Staff have been raising concerns over the transition plan for several months," she said.