The quality of New Zealand men's sperm has halved in two decades - the most dramatic drop of any western country.

New research presented to a gathering of international fertility researchers in Brisbane today was told that the sperm volume carried by the average New Zealand man decreased from about 110 million to 50 million per millilitre between 1987 and 2007.

"It's rather dramatic indeed, and one of the largest seen in studies in other parts of the world," said lead researcher Dr John Peek, of Fertility Associates in Auckland.