Serious cases of abuse against babies have leapt by two-thirds in just two years, according to official figures.

The Ministry of Health revealed 74 children aged under one year were admitted to hospital after violent attacks last year, compared with 45 in 2007.

The surge comes as Social Development Minister Paula Bennett said children were being treated like "dogs".

In an open letter to police, Bennett wrote: "It is unacceptable that our youngest, most vulnerable children are being treated like dogs. The shameful statistics have got to change."

The deaths of 3-month old twins Chris and Cru Kahui in 2006 led to a major Government campaign to stamp out child abuse. Millions of dollars were spent on advertising campaigns such as the Never, Ever Shake a Baby series.

Bennett admitted she was "absolutely horrified" at the findings. "There isn't one nice answer and, frankly, if there was we'd be doing it.

"Some of it's going to be tough stuff for us to get our heads around, like are we leaving children in people's homes too long or putting them back in the right families?"

Child Protection Services chief executive Anthea Simcock said more health professionals were recognising abuse-related injuries than before because of a higher awareness of the problem.

"You have to look at the whole picture, you can't always assume it's one thing," she said.

"One contributing factor could be more awareness.

"Some people will say recession-based stresses in the family could be an issue, some say it's poverty-based."

Simcock said poverty, combined with low education levels in families, lack of support, and younger parents, were also factors.

Detective Senior Sergeant Neil Holden, of the national adult sex assault and child abuse team, said most victims were abused by people they knew. Separate figures obtained under the Official Information Act show the agency charged with protecting vulnerable kids is intervening more than before.

Child, Youth and Family took 85 newborn babies into its care last year, up from 78 in 2006, according to the Ministry of Social Development.

The agency received 1005 warnings about at-risk kids from family members, police or other government agencies last year, more than double the 440 received in 2006.

Ministry of Social Development chief executive Peter Hughes said the number of children under one who die through maltreatment was three times higher than in the 1-4 age group. "Every five days a child under 2 is hospitalised because of abuse."

The police have also come under fire for being too slow to investigate child abuse cases.

Last year, the Independent Police Complaints Authority launched an inquiry into delays in investigating cases after a judge criticised Wairarapa police for slow responses.

It was revealed there was a backlog of 108 files in Wairarapa. Police in Auckland and Christchurch were also battling to clear files, some three years old.

At the time chairwoman Justice Lowell Goddard said the authority had asked police for a robust audit of child abuse files to determine whether delays happened in other districts.

- ADDITIONAL REPORTING: REBECCA LEWIS