"What would concern me is the number of unreported cases. You can only make assumptions about what the level of child abuse and family violence cases would be," Mr Williams said.
"Even if there is a decline - and it's great if there is - the point is that the statistics are still far too high."
CYF recorded 1393 confirmed cases of child abuse in Northland in the 2016 year to June - an average of almost four per day.
The Government said family violence figures were probably five times higher than reported.
Justice Minister Amy Adams and Social Development Minister Anne Tolley recently announced "sweeping reforms" to combat the "horrific" rates of family violence and abuse in New Zealand.
"Young people exposed to family violence... are four times more likely to commit sexual violence against a partner... three times more likely to attempt suicide, and 57 per cent [more likely to] leave school without a qualification," Minister Adams said.
Nationally, CYF found 16,394 cases of substantiated child abuse or neglect in the June 2016 year - 45 each day. Around 8500 cases were emotional abuse, 3600 cases of neglect, 3100 cases of physical abuse and 1200 cases of sexual abuse.
The total figure dropped from 22,984 in 2013.
Child, Youth and Family chief social worker Paul Nixon said it was difficult to definitively say abuse had fallen.
"Child, Youth and Family's focus is not on whether particular numbers go up or down; it is on whether we are making the right decisions when we get notifications."
Mr Nixon said the 17,000 fewer cases requiring CYF action over the past four years - a 28 per cent decrease - were picked up by other agencies.
"That's exactly what's happening, and that was the strategy, to get more [external] agencies involved earlier in the lives of these children."
Amanda Meynell, national manager of a child abuse prevention charity Child Matters, said it's difficult to draw any conclusions from the statistics due to the unknown extent of unreported cases.
She said collaboration between the Government and community was at the heart of the solution.
"While we're talking about the tip of the iceberg, actually that's 16,000 children who have suffered at the hands of people who are supposed to love and care for them.