The restriction is to reduce the chance of accidental incest and to reduce the adverse impact on donors and children if - as the law allows - they seek each other out later in life.
There is also concern among the women about how much Johnson informed them of the number of different women he was making donations to.
Johnson was a 2009 candidate for the office of Governor of Alabama. He then went to work for disaster recovery company Ceres Environmental in Haiti after the devastating quake there in January 2010.
He moved to Christchurch after the February quake without his wife Kathy, a two-time Mrs America finalist who has three children from a former relationship. Johnson is long-term recovery manager for Ceres NZ.
The Herald on Sunday approached Johnson on Thursday at a restaurant in Christchurch where he had just finished dining with one of the women he had successfully impregnated.
He said the urge to become a biological father was "a need that I have".
"I am married to the most beautiful woman in the world. When I married her I knew we couldn't have any more children. She had a hysterectomy 10 years ago.
"There is nothing my wife would want to give me more in the world than a child of my own."
Johnson, who has a degree in chemistry and is a member of the international high-IQ club Mensa, said he had told his wife he wanted to act as a sperm donor. The couple had sought counselling shortly after. He said he left believing she was aware he was going to continue.
Asked if his wife knew he was donating sperm, he said: "She does now."
He said she did not know of any of the pregnancies. He said he had not planned on telling her until the children were born.
"Every person who is a father and a mother knows why I am doing this. If life's circumstances had dealt me a different hand I wouldn't be doing this. It is not the hand that life has dealt my wife.
"Reproduction and having children is as basic a human need as eating."
Johnson said his urge to biologically father children came after the arrival of Kathy's first grandchild in April.
"That's what did it to me. When I held that grandbaby, that's when it came to me ..."
Johnson returned to New Zealand and enlisted in the online sperm donor registries.
The women spoken to by the Herald on Sunday say they met him through different sites. They included a number of women in same-sex relationships.
In his political life, Johnson campaigned on a conservative Christian platform which opposed gay marriage.
He said he did not know the "relationship status" of the women he donated to: "I just know they want to have children." Asked if it mattered, he said: "I'm not going to answer that question."
Johnson said he wanted to be involved in the lives of the children he had helped create if it was welcomed by their mothers. "I'll be just as much a parent to them as they want me to be, as I am to my own children."
He said publication of this story would make it difficult for him to stay in New Zealand.
"I've been trying to get my wife over here, my family over here ... so I can be around for these children."
He asked: "Would it make any difference that from the first month (of pregnancy) I started sending [one of the women] money to help?"
He said he was also financially supporting another pregnant woman. The payments he made in that case were increased because the woman already had one child.
Johnson also said he was aware of the fertility community's recommended limit of four families for each donor. He believed he had stayed inside that limit.
Our inquiries into Johnson's online life began after personal details were sent to the newspaper from an internet-based account under a pseudonym.
When approached for comment, Johnson confirmed the accuracy of the details.
On Friday, Johnson changed his mind about speaking to the newspaper and said he did not want his comments published.
He demanded to know the newspaper's source, alleging the newspaper was in receipt of information that must have been sourced through phone or internet hacking.
Yesterday, Johnson said he had made a sworn statement to the police alleging his personal information had been illegally obtained.
He said some of the women to whom he had donated sperm were unaware of the extent of the donations.
He said he had not intended making the women aware of the existence of other siblings until they had babies, at which point he would tell them.
Johnson said he was aware of a requirement for the names of donors to be stored centrally so children could access information about biological siblings.
He said the requirement did not exist until the children had been born - and as yet, his biological children had not been.
The Herald on Sunday did not and would not use information obtained from illegally intercepted communications.
Complex fertility
There is no easy path for those seeking fertility help.
There are fertility clinics, which enforce a strict screening code and set of rules, which include not accepting donors aged 45 or over.
Outside the clinics, there is the unregulated path of community and online donors.
Those in the fertility medicine community say any decisions have to be made with the children foremost in mind.
The Human Assisted Reproductive Technology Act sets that consideration in law.
The law allows children at age 18 to ask the fertility clinic or the registrar of Births, Deaths and Marriage for the identity of the donor and any siblings conceived using the same donor.
The legal rules are reinforced in the fertility medicine industry by voluntary guidelines.
Among those is the rule that a donor not make sperm available to more than four families.
Canterbury University social sciences professor Ken Daniels said there were concerns that high numbers of children from one biological parent could have psychological repercussions.
He said it placed stress on donors and their offspring when they finally met years later.
Daniels specialises in fertility medicine and has served on the national Advisory Committee on Assisted Reproductive Technology.
Outside the clinics, there are binding agreements which would-be mothers and donors can sign.
The unregulated market meant "if it starts to go wrong, it becomes very complicated", Daniels said.
"The drive to become a mother has to be put alongside the implications of using this particular method.
"[They] have the potential to be held up as examples of where it can go wrong."
