Startling figures on a sudden drop in the sperm counts of New Zealand men are being questioned by the very group that did the study.

The research by Fertility Associates suggests sperm volume carried by the average man dropped from about 110 million to 50 million per millilitre between 1987 and 2007. But the figures are not reflected in most other studies in the western world.

Fertility Associates researcher Dr John Peek says the study raises more questions than it answers.

"Because we've looked at potential sperm donors as giving the information, it could be real or there could be something really subtle about what sort of men are volunteering to be sperm donors. But it's too big a change to ignore and say, well, something else might be the cause."

Dr Peek says it is hard to get semen samples that are representative of the population, so researchers have only soft data to work with.

Dr Peek says there are two theories about declining sperm counts. The problem could be due to modern environmental factors such as pollution, or it could be happening to baby boys before they are born as a result of influences such as a mother's diet. However, the biggest impact on fertility rates is still women waiting until later in their lives to have a baby.