The teen pregnancy rate hit a 40-year peak in 2008 despite a long-term downward trend in the proportion of teenagers actually giving birth.

A report by Social Development Ministry analyst Dr Barbara Collins has found that an almost continuous rise in the number of teenagers having abortions has more than offset the downward trend in the actual birthrate since the mid-1980s.

Both the abortion rate and the teenage birthrate dropped last year, but the overall teen pregnancy rate was still the fifth-highest since the teenage birthrate peaked in the early 1970s.

The teenage birthrate was also still fifth-highest in the developed world at last count, after Mexico, Turkey, the United States and Bulgaria.

A campaigner against the domestic purposes benefit, former Act Party candidate Lindsay Mitchell, said the long-term upward trend was a concern because research showed that children of teenage mothers often did worse than other children on educational, economic and health measures.

"All I'm worried about is the evidence that shows that these children are at risk, and it also robs young girls of a time in their lives when they should be laying down the foundation for a stable relationship and a career if that's what they want," she said.

But Child Poverty Action Group economist Susan St John said Mrs Mitchell was on a moral crusade and should welcome the fall in the teenage birthrate. "What is the solution to this - more abortions?" she asked.

"What is happening is that teenagers are choosing abortion. They shouldn't have been brought up in a sexualised environment where this is all happening. They should have had adequate counselling.

"Women reach their peak fertility in their teenage years. That used to be when women had babies, so it's not surprising that there's a fair few of them having them."

Dr Collins' data show that the teenage birthrate dropped from a peak of 67.6 for every 1000 girls aged 15 to 19 in 1972 to a postwar low of 25.6 in 2002.