"Things like increased access to long acting, reversible contraception, which is making it easier for women to control their own fertility.
"But I don't think anybody really knows."
She said there needed to be more research done to find out the reason for the drop and why women were choosing whether or not to terminate their pregnancies.
A main barrier to having an abortion was that it was still in the Crimes Act, Dr Healey believed.
At the moment, in order to qualify for an abortion, a woman needs the approval of two certifying consultants.
The doctor performing the abortion has to believe it is immediately necessary to save the life of the patient or to prevent serious, permanent injury to her physical or mental health, or has to believe the foetus would have a serious disability.
Family Planning chief executive Jackie Edmond said New Zealand's trend was following international figures.
"It's about more reliable contraceptives ... and more accessible ... and that seems to be one of the main things internationally - access."
Sex education was also reaching more young people, she said.
"But it's hard to say exactly why, in terms of a New Zealand perspective."
Family First NZ said the abortion rate would continue to drop as knowledge of the prenatal development of the unborn child increased, and as an increasingly pro-life younger generation become parents.
"The 'bunch of cells' argument which has driven the right-to-abortion argument is now just 'flat earth science'," Family First said.
"As the average person has learnt more about the biology of prenatal development, they have become more 'pro-life'," spokeswoman Marina Young said.
The Green Party earlier this month announced it wanted to make abortion freely available to women up to 20 weeks pregnant.
By the numbers:
* a continued drop in the abortion rate (abortions per 1000 women) to 15.4 (down from 16.1 the previous year), its lowest since 1994.
* a continued drop in overall abortion numbers at 14,073 (its lowest since 1995, down from 14,745 in 2012)
* a continued downward trend in abortions among the very young (11-14 year olds) and teenagers (15-19 year olds)
* most abortions - 64 percent - were a woman's first.