It is understood he signed paperwork giving his mother ownership rights to the sperm if he died.
In 2010, seven years after he died, the family tried to fulfil his wish and applied to the Ethics Committee on Assisted Reproductive Technology (Ecart) for "the posthumous donation of sperm from the deceased to his sister's female partner".
But the application was rejected because it contravened guidelines for the storage, use and disposal of sperm from a dead man.
Under current laws it can only be used with prior written approval from the man.
Duncan was also a minor when he died. Legally no one has the right to use the sperm stored by a minor except the person himself.
It is understood the Duncans will argue Cameron intended for the sperm to be used when he gave his mother ownership rights.
The case is in the public eye after the family applied to Ecart to extend the storage time of Cameron's sperm as it had reached the 10-year limit.
Ecart said it could not grant an extension for something that was illegal, but accepted the Duncans could challenge the decision.
The committee gave the family one year to "allow the applicant to take any steps she thinks legally appropriate to apply to Ecart for use of the sperm".
Duncan family members declined to comment but it is understood they are taking legal advice in the hope of carrying out Cameron's wishes. They have until November to plead their case.
Otago University professor Grant Gillett said the family could avoid a court process if they could present expert ethical opinion.
"They could go to court or they could get supporting evidence from the type of expert witnesses the court would call," he said.
"If these experts were prepared to say it was the kind of thing Ecart could approve Ecart might allow them to go ahead."
Gillett said there was no legal precedent so the court would call expert witnesses and the magistrate would take those opinions seriously.
In cases overseas, women had been granted possession of their dead husband's sperm and had travelled to countries where it was legal to use it to conceive a child without written consent.
Duncan became a household name for more than his public cancer battle. He was outspoken about the need for more organ donations and represented Auckland in softball.
His well-known and touching film DFK6498 - his national health index number - compared hospital treatment for cancer to a relentless prison sentence.
He counted filmmaker Sir Peter Jackson and late broadcaster Sir Paul Holmes as friends.
