Ken discovered he fathered 11 children. Now, he has an excruciating message for them all.

Four years ago, a phone call turned Ken Allen’s life upside down.

Ken, a now-retired teacher, was on his lunch break in late 2014 when he received an unexpected call from an IVF clinic.

Before the call, he had four children. After the call, he was father to 15.

Ken and his wife Sue, from South Australia, have been married for 36 years. Together they have three biological children and one adopted daughter.

Speaking to Sunday Night, Ken said his donor journey began in the late 70s. He was studying to be a midwife and witnessed many families who struggled to conceive, so he asked Sue her thoughts on him becoming a donor.

Sue worked at the same hospital and had similar interactions with families, so she did not hesitate in supporting him.

Ken made multiple anonymous donations over two months. The information a potential couple would receive about Ken was very limited.

In a section asking donors about their philosophy on life, Ken wrote “Man cannot discover oceans unless he has the courage to lose sight of the shore.”

“I knew that there was a possibility that my donor offspring may in the future get a hold of this, and I really wanted it to encourage them to pursue their dreams, and also to perhaps pursue me,” he told Sunday Night.

But even so, Ken never expected contact. Everything was kept anonymous and donors were never informed about whether any pregnancies had come from their donations.

Decades later, Ken received a call to tell him he had 11 donor children – two girls and nine boys. One of the daughters was looking for him.

Chloe Allworthy, 25, had spent seven years searching for her biological father. During her search she had come across Robert, 27, whose parents had also used Ken as a donor, making Robert her half-brother.

For much of their lives, all they knew of their biological dad was from the two-page donor summary.

"He wrote something which I presume at the time meant a lot to him, which was a quote which said, ‘Man cannot discover new oceans unless he has the courage to lose sight of the shore.’ For a long time, that was all I had in relation to the donor. It stuck with me a lot, so I got that tattooed onto my body after a lot of thought," Robert told Sunday Night.

Ken has now also met his other daughter, Melissa and another son, Nathan.

He has seven other sons out there somewhere, and Ken has a very difficult reason for wanting to find them.

"The last four years, I’ve been diagnosed with symptoms of Parkinson’s. There are a number of other medical issues that have a strong hereditary basis, and they’re starting to show up in some of our children now, so for them to be in their 20s and 30s, they need to be aware of those medical issues so that they can actually take good care of themselves and seek the appropriate treatments.”

For now, Ken is enjoying spending time with his new expanded family.

"It’s been pretty overwhelming, but just I’ve felt nothing but joy and happiness and love from these guys."