For the last 35 years, Kerryn and Stephen Longmuir have dealt with a never ending cycle of nappy changes, growth spurts, weekend sport and teenage angst.

Key points: Kerryn and Stephen Longmuir have fostered more than 400 children over 35 years

Kerryn and Stephen Longmuir have fostered more than 400 children over 35 years They say their passion is yet to wane, but advocates say carers are underpaid

They say their passion is yet to wane, but advocates say carers are underpaid Victoria is currently experiencing a shortage of foster parents

Most importantly, they have provided a home to broken children and given them chance to smile and feel safe.

About 400 foster children have passed through their house in Greensborough, in Melbourne's north-east.

"Most of the kids are coming in really damaged," Ms Longmuir said.

"They don't have a bed, they don't have food, they don't know what a bedtime is."

Ms Longmuir said many children who came into their care were exhausted and traumatised, having been removed from parents caught up in domestic violence, drugs and crime.

One of the most upsetting sights is babies who have inherited ice addictions from their breastfeeding mothers.

The only way to calm them is with cuddles and a lot of love.

"They don't settle easy and they get the jitters," Ms Longmuir said.

Some of the children the Longmuirs have taken in have stayed for a night, while others have lived there for years.

Sometimes, as per a court order, they leave with only a few hours' notice.

Kerryn Longmuir shares a laugh with Jess, Andy and Valentina Pham, siblings who are in her permanent care. ( ABC Melbourne: Kristian Silva )

'There's never any swearing here'

The Longmuirs believe turning a child's life around starts with a few simple steps.

"They've got to sit at the table to eat," Ms Longmuir said.

"There's never any swearing here.

"Once they're here and they see what a normal family is like, then they start to do those things."

The Longmuirs' dining room wall is filled with pictures of their immediate and extended family, including Kerryn and Stephen's biological children and three others who are in their permanent care — siblings Andy, Valentina and Jess Pham.

There is never a shortage of work for Stephen Longmuir at home. ( ABC Melbourne: Kristian Silva )

Pushing through the pain

When Ms Longmuir's second child was born with a disability she decided it would be her last.

But her passion for parenting was reignited when she saw a flyer about foster caring at a local community fair.

"It started out as one child and it went so well, that I thought 'I can do this,'" she said.

"They just kept coming and coming."

Husband Stephen is just as committed to the cause.

When the ABC visited the family home last month, Mr Longmuir was busy bottle-feeding a two-month-old girl who was starting to sprout tufts of ginger hair.

The girl was a few weeks away from being adopted to new parents.

"This is the best part, when she goes in a couple of weeks," Mr Longmuir said quietly, as the baby fell asleep in his arms.

"When new parents come and pick up a baby and they've been trying for 20 years and they've finally adopted … fathers are just overwhelmed, they can't handle it."

Mr Longmuir, who used to work as a floor tiler, is lucky to still be alive.

Twelve years ago he spent six months in a back brace after falling through a ceiling, fracturing his back, pelvis and arms.

"The first thing I could think of is, 'I'll lose all the kids,'" he said.

Foster parents underpaid, advocates say

Katie Hooper says foster carers are underpaid and facing increasing pressure with complex children. ( ABC News: Matilda Marozzi )

About $4.8 billion was spent on child protection and out-of-home care nationally during 2017-18, according to the Australian Institute of Health and Welfare.

Despite regular increases to funding, statistics show foster families are increasingly calling it quits in Victoria and Queensland.

AIHW statistics show that 606 Victorian families exited foster care in 2017-18, while only 375 joined.

Katie Hooper, the chief executive of the Victorian Foster Care Association, said a busier society, an increase in children with complex problems, and inadequate carer allowances were to blame for the shortage of carers.

Victorian parents receive between $395 and $580 a fortnight for children classified as 'level one' by the Department of Human Services.

More complex cases, such as children with major behavioural issues or disability, attract higher compensation.

Ms Hooper said families needed at least an extra $100 a week.

Shortage leaving kids at disadvantage

Jenny McNaughton, a services director with the charity Berry Street, said a shortage of foster parents meant some children were being placed in residential care homes when it was not suitable for them.

"They're living with three other children who also have complex behaviours," Ms McNaughton said.

"So in many cases, it doesn't actually help resettle them."

Ms McNaughton wants to see professional foster carers paid an average wage to look after children full-time, and believes this model could provide a boost to numbers.

Ms Longmuir said red tape has also been a major stumbling block over the years.

After more than three decades in the foster care system, the Longmuirs are used to questions about when they will eventually call it quits.

"I still love it," Ms Longmuir said.

Mr Longmuir said they still have the passion to help more children.