Fertility Associates group operations manager Dr John Peek said there were 10,600 embryos frozen in liquid nitrogen banks across the group's four clinics in Auckland, Hamilton, Wellington and Christchurch.
At least two other fertility clinics, Fertility Plus at Auckland's Greenlane Hospital and Otago Fertility Services in Dunedin, also held embryos which would be affected by the deadline.
Of those with Fertility Associates, Dr Peek said 1950 had been identified as approaching the 10-year or longer limit, with the embryos belonging to 350 couples or women.
Six women or couples had used embryos that were eight to nine years old to have a child.
In 2011 an estimated 35,000 couples underwent 70,000 cycles of IVF treatment in New Zealand and Australia, resulting in 13,000 live births.
Dr Peek said those figures continued to increase and last year Fertility Associates froze 2730 embryos about 230 a month thawing around 1700 for replacement.
Owners of the embryos and gametes affected by the deadline must apply by May this year to the ministerial Ethics Committee on Assisted Reproductive Technology if they want to extend storage, and approval is not guaranteed.
For Fertility Associates the challenge is contacting the hundreds of people affected by April, many of whom have moved.
Dr Peek said there was also concern over being able to contact young men who might have stored sperm as teenagers during cancer treatment and who might be overseas and not thinking about starting a family.
He believes half of those who had embryos in storage would find the looming decision incredibly difficult while the other half probably expected the clinic had already disposed of their embryos.
"There's those that maintain contact with the clinic and are still paying for storage [$250 a year] and I think for them it's the hardest because they are obviously still thinking about a future for their embryos," he said.
"But for some people their family's complete and I guess the remaining embryos don't weigh heavily on their mind."
Dr Peek said the main barrier deterring people from donating embryos was that they saw the embryo as a full genetic sibling to their children.
"Having an embryo which is yours and therefore the child is genetically yours, it's like adoption in that your genetic investment is the same. People have a lot more feelings of protectiveness and identifying with the child than if they donated sperm or eggs. It's a step too much for some people."
Under the law, Fertility Associates must begin discarding the frozen gametes and embryos after November 22 if the owners have not responded to the efforts to contact them, which will include letters, phone calls and checking electoral rolls.
Unlike in Australia, embryos cannot be donated for research, which means if couples do not want to extend storage they must make one of the biggest decisions of their infertility journey.
The implications of that deeply personal decision - one that must be agreed by all parties involved - are huge and include questions about if the child became sick, would the donors feel obliged to help.
Fertility Associates counsellor Sue Saunders said there were many reasons for storing embryos including couples putting off the inevitable.
"People store embryos because primarily they've finished their family. They store embryos because they've had a child and they see their embryos as potential children and can't either morally or philosophically or religiously dispose of them."
Indecision over fate of embryos
Rebecca was 35 when she started trying to conceive.
It was five years before she was diagnosed with endometriosis and she and her partner underwent IVF.
The couple made seven embryos but after four replacements there was still no baby.
Rebecca, who does not want to be fully identified, had surgery to remove the endometriosis and within a month she fell pregnant naturally, giving birth to a daughter, now aged 2.
But the then 43-year-old suffered the deadly pre-eclampsia - high blood pressure more common in older women - and was told there was a real risk it would return in further pregnancies.
"I'm going to be 46 in September so we've decided no, we won't have another one. It was a really big, hard decision because obviously you would like to give your child a sibling."
Rebecca said the three remaining embryos in storage have been there for six years because the couple cannot decide their fate.
"At this stage we can't make the decision so we're still paying.
"It's not that they're not wanted but am I being fair having them given our age and with the health risks? So it's awful. The technology is fantastic but there's a price to pay and you have to go into it with [your] eyes open."
Embryo storage
* In New Zealand, women undergoing IVF are encouraged to replace one embryo at a time
* The spare embryos are frozen in liquid nitrogen
* They can be thawed at any time for replacement or donation (to try to form a pregnancy)
* Not all survive the thawing process
* They can be stored for 10 years, after which permission must be given by the Ethics Committee
