When John Guthrie and Dennis Cash announced their plan to foster teenagers almost 20 years ago their friends thought "they had rocks in their head".

Why would two men without kids want to take on the challenge of a traumatised teen? And what of people's perceptions of a same-sex couple fostering?

Dennis shared some of these concerns.

"Initially I wondered whether we really had the ability to deal with young children that might have difficulties and need more specialised care," he said.

"I had some concerns about fostering young boys, being a same-sex couple, only because of what people might think.

"So my stipulation was that they had to be girls."

Dennis Cash (far left) and John Guthrie (far right) have fostered and mentored 12 teenagers in the past 20 years. ( Australian Story: Sarah Gerathy )

According to Barnardos Australia, most people who foster children want babies, but John and Dennis opted to take on teenage girls under the child protection charity's permanent placement scheme.

They agreed to take girls from the age of 12 until they turned 18 and two decades later their family is flourishing.

The couple, who have been together for more than 30 years, has fostered and mentored more than a dozen children.

But it is only since they started filming with Australian Story that John and Dennis have learned the full extent of the trauma that the children in their care had experienced.

When each child arrived they were given a briefing, but their main focus was on the child's future rather than their past.

John is pleased that the young women now feel confident enough to open up, not only to their dads, but to the world at large.

"We have known these kids for years and they told us very little about what they had been through," he said.

"Now they are adults they feel they can talk and I think it has been really good for them to own their stories."

'She liked to break my bones': Sarah's story

Sarah lived in 10 foster homes and refuges before she settled in with John and Dennis as a teenager. ( Australian Story: Sarah Gerathy )

Sarah, now 30, was the first child Dennis and John fostered.

"I was lucky to walk out of my biological family's life alive," she said.

"I don't think I could have survived any longer with the level of abuse."

By the time Sarah arrived at Dennis and John's house, she had been admitted to hospital multiple times and suffered 21 fractures.

"My mother had Munchausen by proxy. She liked to break my bones."

Despite cries for help, Sarah said nobody believed her until a Department of Community Services worker took up her case.

Sarah started living with John and Dennis when she was a teenager. ( Supplied )

After 10 different placements, Sarah, then 12, arrived at Dennis and John's home and immediately took to her new environment.

"There was no yelling, no screaming; it was just peaceful," she said.

But after a short honeymoon period, John and Dennis said they had their first taste of the realities of fostering. John recalls Sarah's first day at school.

"She put her uniform on, I thought everything was rosy, I took her to school and then she bolted and I ran the streets looking for her," he said.

Sarah had missed years of schooling and was terrified.

"I didn't feel like I could fit in. I had missed out on way too much," she said.

While Sarah was worrying about fitting in at school, John and Dennis were wondering whether they could cope. And it was starting to impact on their relationship.

"I was thinking maybe we could have done this or that and John and I would disagree on what we should have done or didn't do," Dennis said.

After reassurance from their case worker, John and Dennis persevered and Sarah soon settled down.

John said while a lot of children in care have trust issues, patience and love is the key.

"I made their lives miserable but they loved me. With all my flaws and with all my baggage, they loved me," Sarah said.

'It's a very vulnerable situation'

Musu was born in Sierra Leone, West Africa, and came to Australia as a refugee when she was eight years old. ( Australian Story: Sarah Gerathy )

By the time Musu arrived at Dennis and John's at the age of 12 she had survived a civil war in Sierra Leone and severe beatings at the hand of a family friend who brought her to Australia as a refugee.

After alerting the police to her predicament, Musu was placed in the foster care system.

Musu recalled her delight when John and Dennis first came to collect her in their black convertible.

"Is this for real?" she remembered thinking.

"Are they my carers? They're playing a joke on me."

Australian foster families As at June 30, 2016, 46,500 children were living in out-of-home care

As at June 30, 2016, 46,500 children were living in out-of-home care Almost 90 per cent of children in out-of-home care are in foster or relative care

Almost 90 per cent of children in out-of-home care are in foster or relative care About 10,000 foster care homes have more than one child placed with them Source: AIHM Child Protection Australia 2015-16

But within days, John and Dennis were back in familiar territory. Musu would barricade herself in her bedroom shoving furniture up against the door to keep her new foster carers out.

She said it was difficult in the early days of moving in.

"It's very hard because you don't know these people; you are moving in with strangers. It is a very vulnerable situation," Musu said.

Over time, Dennis and John won her trust and she settled in.

Musu's advice for other children who find themselves in foster care is to let go of the past and make the most of your new life.

"I know that there are a lot of kids who have gone through traumatic experiences and it takes time to move on," she said.

"But you really have to take what your carers are giving you and accept the help that's there."

Opening the home for more children

Long-time partners Dennis Cash and John Guthrie are hoping to foster a newborn soon. ( Australian Story: Sarah Gerathy )

Musu described John and Dennis as "extraordinary".

"I speak from my heart because of my experience and what they've done for myself and other kids, just the amount of love and attention — they're great people and I would give them the world," she said.

Although John and Dennis are in their 60s, they say as long as there are kids who need fostering they will be there to take them in.

For the past two years they have been looking after 14-year-old Hailie and later this year they plan to foster a newborn baby.

Dennis said that while being a foster parent is challenging, it is also a rewarding experience.

"The reward for us is to see them come back, so the door is always open and they are always coming back," he said.

When John speaks about the children he glows with pride.

"Every day we live vicariously through what they're doing. I love it when we're all together," he said.

"I really can't imagine my life 20 years ago without having these children."

Australian Story's 'Modern Family' airs at 8:00pm on ABC TV and ABC iview.