The Independent Children's Monitor says Oranga Tamariki is failing to do enough checks on children returning home from state care.
The ministry's latest Safety in Care report shows 9 percent or just over 500 children in its care were abused and neglected in the year ending June 2023.
That is a significant jump since the ministry's first report in 2019, when almost 6 percent of children in Oranga Tamariki's care were found to have been harmed.
The level of abuse was worse in residential care and for children returning home.
Thirteen percent who returned home were harmed, with half suffering emotional abuse and around 26 percent assaulted.
The monitor's chief executive, Arran Jones, said the watchdog's own investigation showed not enough was being done to ensure children were safe.
"What we saw is that there wasn't sufficient presence from Oranga Tamariki, they weren't there in these homes to make sure that everything was being done to make sure they were safe. So only 35 percent of these kids were visited weekly for the first month from when they returned. You'd expect to see the rate of visits a lot higher," Jones said.
The children's watchdog is also accusing Oranga Tamariki of sending children home when the parent is not ready for them.
Jones said children going home prematurely were not going back to a stable environment
"We heard from some of them that said they just weren't ready for their children to come home, they hadn't completed programmes that they've been wanting to do whether it's to do with drug or counselling. And so actually the whole system needs to step up here. There's a lack of services and support for these families," Jones said.
Meanwhile in youth residences about 20 percent of children were abused in the last year.
From these figures 84 percent of the harm experienced was caused by other children in the facility, while the other 11 percent came from staff members.
The report showed there were more than 400 cases of physical abuse, almost 300 of emotional abuse, 95 of sexual abuse and 67 cases of neglect.
Jones said the independent monitor would release its own report on Thursday looking at what was happening to children in care.