Tara Wales said her almost four-year-old daughter Acacia recently walked the Six Foot Track in three days - a distance of 46 km and 5,200 feet elevation from Katoomba to Jenolan Caves. And she said she and her husband Samuel didn't need to carry her once.

When they got to Jenolan Caves House, a staffer commented that at three years and 11 months, she may be the youngest to ever walk it.

It's not the first time the young hiker has hit a milestone.

As a toddler she walked with her parents along the 62 kilometre Trans-Catalina Trail in Southern California and made the front page of a local paper.

On that trip the family told the Catalina Islander newspaper that they "made friends with other hikers and Acacia knew she would get to see those people again at the next stop. Despite getting caught in a rainstorm, a fall and few minor meltdowns, Acacia came through like a veteran, finishing a day ahead".

The now Sydney-based parents met while hiking in 2014 - so naturally wanted to include their daughter in their outdoor adventuring.

"I was 28 and decided to leave my job in LA, working on farms all around Europe and I started writing articles," Mrs Wales said.

"I was writing about hitchhiking and he read it and we started an emailing repartee for four months. We fell in love via email.

"I invited him to New Zealand, we fell instantly in love, two weeks later he proposed and we fell pregnant on the honeymoon. Cachi didn't give us any time, she wanted to get going. We vowed this [being parents] wasn't going to stop us, we would keep travelling and keep hiking."

Mrs Wales said they always took their daughter in a backpack but once she could walk, they got her used to short hikes.

"It took many many months, meltdowns and more patience than we ever knew," she writes in her blog.

On the 6 Foot Track they kept her happy by playing audio books and podcasts, bribing her with snacks, and singing Christmas songs.

"Acacia seemed stoked to be on a swinging bridge in the rain! I on the other hand, gritted my teeth in sheer panic the entire time with her in front of me. I actually had her count her steps out loud so she wouldn't go too fast and create too much side momentum," Mrs Wales said.

They said the biggest challenge was at the end. With cave tours closed due to COVID and no public buses, Acacia's dad Samuel, had to hitchhike back to their car at Explorers Tree, returning six hours later.

The trio is hoping to do the Pacific Crest Trail - a massive 4,270km from Mexico to Canada - with peaks above 10,000 feet next year if COVID allows. How do they think she will go?

The family defend those who "consider it abuse".

"We've had people who considered it abuse ... I think we know our child enough to know when to stop. We are always asking her 'Are you enjoying this?' It's just the people we are, we are a family of hikers."