The service, which is based at Dunedin Hospital and covers Otago and Southland, only had four or five men donating sperm each year, associate professor Wayne Gillett said.
"We need eight to 10 donors in a place like Dunedin.
"It does require the generosity of men who will donate sperm but it's not an easy thing to do."
Prof Gillett said a law change in 2004, which stated that donors had to be identifiable, put many men off donating sperm.
"If children wanted to know, then the donor had to be identified."
Prof Gillett said the rigorous selection process, which included counselling and health checks, was another aspect of donation that might put men off.
"One in four men would be suitable donors.
We don't have enough [but] we would only want donors to donate if they genuinely wanted to help."
Each donor's sperm could only be used for four families to maintain genetic diversity, and each donor had to provide a profile to allow people who needed donations to pick the donor they wanted, Prof Gillett said.
"Donors write themselves a profile where they describe all their interests and medical history but they are not identifiable.
Often it [the decision] is based on height and build and things like that."
Sperm donation was more lucrative for donors 30 years ago as they were paid $25 for each sample, of which 10 or more were needed, Prof Gillett said.
There was no shortage of donors then, he said.
Nowadays sperm donors are only reimbursed for travel and parking costs.
By law, donated sperm can be kept in sperm banks for no more than 10 years.
