A Dunedin case where a man and his daughter had a child together has highlighted the phenomenon of genetic sexual attraction - but they're not the only ones in New Zealand experiencing these feelings, a sex therapist says.

More and more cases of a similar nature are coming to light, where siblings or parents and their offspring meet for the first time in years and end up developing a sexual attraction to each other, director of Sex Therapy New Zealand Robyn Salisbury said.

A father and daughter were sentenced to community work and supervision in the Dunedin District Court earlier this week, after knowingly entering into an incestuous relationship, the Otago Daily Times reported.

They were warned by a judge to end their relationship or face prison if they refused.

The 32-year-old man and 18-year-old woman admitted to the incest, but said they had fell in love after they started living together as husband and wife.

The pair had only met in 2010, when the man, who fathered the girl when he was just 14, made contact with her for the first time. It is understood that they had not had a relationship prior to that.

Some children who meet their siblings or birth parents at a later stage in their lives may develop a strong sexual attraction to their relative, and vice versa, Salisbury said.

The psychologist worked with a number of people in similar situations and said cases of genetic sexual attraction were becoming more prevalent.

That's because families were becoming increasingly fragmented - solo parents and separation were more rife - and changes to New Zealand's adoption law in the mid 1980s allowed for most adoptees and birth parents to access identifying information about each other and track one another down, Salisbury said.

"There are more people meeting up with previously unknown family members and being completely overwhelmed with how powerful that can be."

It was a hugely emotional experience to find a lost relative, prepare to meet them and then actually come face to face with them, she said.

"Very strong feelings are experienced. For some people those powerful feelings and the kind of depth of connection that can happen are most readily experienced and expressed sexually. It's the whole 'falling in love' scenario that people only know one track to go down in order to express those feelings."

While genetic sexual attraction wasn't a universally-accepted diagnostic category, it was experienced enough for people to have become aware of it, Salisbury said.

There had been a number of cases involving genetic sexual attraction highlighted in recent years.

John Deaves and his daughter, Jennifer, were both married when they were reunited after 30 years apart in 2000.

A sexual relationship developed and they ended up leaving their spouses to live together as a couple.

They went on to have two children, one who died when just a few days old.

They both admitted incest and were sentenced to three years' good behaviour bonds in 2008, but they said at the time that they did not view each other as father and daughter.

"After I had been there quite a few days I started to notice my feelings were changing and I was seeing him as a man, as a person, who was loving (and) caring," Jennifer Deaves told local media at the time.

In another case, a German man, who was adopted at age four, fell in love with his biological sister after meeting her for the first time in 2000. The couple went on to have four children before being convicted of incest.

- © Fairfax NZ News