It was important that services did not deteriorate during the changeover between CYF and the new ministry, he said.
"The new agency must not drop the ball. It has to ensure these children thrive. We need the new agency to listen to them and give them the best possible chance in life."
High levels of re-abuse and re-victimisation prompted the review by an Expert Advisory Panel in 2015.
The panel found the existing system to be fragmented, lacking in accountability and without a common purpose. Children in care had poor long-term outcomes in health, education, employment and in living crime-free lives.
About 230,000 New Zealand children under 18 are at significant risk of harm during their childhood, and about six out of 10 of this group were likely to be Māori.
Each year about 60,000 children were brought to CYF's attention, and at any time about 4900 New Zealand children were in statutory care.
Record numbers
New figures released on Wednesday, however, showed that the number of children in CYF custody had now risen from 5159 to 5453. That included an 8 per cent rise in the number of Maori children in care.
The trend showed that child services were underfunded in New Zealand, the Green Party said.
"We haven't seen evidence that the new Ministry for Vulnerable Children has the resources and funding to deal with existing caseloads properly let alone increased numbers of children in care," social development spokeswoman Jan Logie said.
Logie said the number of youth justice beds had been reduced since 2014 despite increasing demand, and children were being locked up in police cells instead.