Have you ever heard of slacklining?

It is a modern form of tightrope walking that uses tension to create a kind of long and narrow trampoline between two fixed points.

You may have seen people doing it this summer in parks or beaches across Sydney, but one group has taken the sport to new and spectacular heights.

Guilherme Lopez, Travis Foster, Luke Sarantos and Chris Wallace are all experienced slackline walkers. Recently they tackled the Kanangra-Boyd National Park 180 kilometres outside of Sydney.

Attaching the line between two trees above a waterfall in the park, their feat was captured in the short film Slack.

Nick Erickson, a Sydney-based camera operator, was one of two people that tagged along to document the boys' adventure.

He said he wanted to create a compelling story about the highly qualified walkers.

Filmmaker Nick Erickson was one of the shooters behind the Slack documentary. ( Jono Blake )

"I can't do it very well myself, but I know how to use a camera so I came along as part of the team," he said.

On the two-day shoot an entire day was dedicated to ensuring the slackline equipment was properly set up.

Each walker used a harness and leash when attempting to cross.

Mr Erickson said watching someone try to walk 100 metres above the ground is frightening, despite knowing the safety precautions taken.

"As soon as the first person started, I thought, 'please [let] nothing go wrong ... I don't want to film someone die'," he said.

A close call behind the lens

While Mr Erickson did not attempt to walk across the line himself, he did have a close call while shooting on the side of a cliff.

"I was filming and looking through the waterfall and almost slipped," he said.

"I wasn't in danger but it woke me up."

Mr Erickson said that while slacklining was undoubtedly hazardous, it was more about calculated risks and not taking any chances.

"They seem crazy, but they're also totally conscious of what they're doing," he said.

"You're putting your life in your own hands so you want to make sure you're safe."

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Mr Erickson said the five-minute short is both calming and nerve-wracking, as the natural beauty of the national park dwarves the human figures crossing the void.

And rather than try to direct the walkers, the filmmakers simply captured what happened.

"We spoke with the guys about what the plan would be and then we focused on the theme of the story and the practicalities," he said.

"Luckily every angle you could pick looked good."