Johnno Johnstone was known to say that he was the only man to walk across Bass Strait.

He didn't actually cover the entire 500-kilometre distance but he did walk — and at times was bounced and dragged — for 43 kilometres across the sea floor that separates Tasmania from the Australian mainland.

It was 1938, global war was in the wind, and the only communications link between Victoria and Tasmania — a telecommunications cable across the strategically important Bass Strait — had been damaged.

All that was known was that the cable was damaged somewhere between King Island and Victoria.

John Edward Johnstone OBE, who preferred to be known as Johnno Johnstone, was an accomplished deep-sea diver from the United Kingdom and was based in Devonport in Tasmania at the time.

His six weeks of diving in the Strait required incredible bravery, nerve and some ingenious improvisation.

A busy wharf scene in 1940s Devonport. Johnno Johnstone launched his Bass Strait dive mission from here in 1938. ( Supplied: Robinson Collection )

Johnstone spent so much time underwater that he befriended a family of fur seals — a side story revealing his great love of nature but not blessed with a happy ending.

Johnstone walked, hopped and skied along the bottom of the Strait, and often sat on a large hook that was towed by the ship.

He simultaneously lifted the cable just above the sea floor where he could check it for damage.

Art class unearths forgotten history

A diving suit similar to one worn by Johnno Johnstone. ( ABC Northern Tasmania: Rick Eaves )

The Bass Strait Maritime Centre (BSMC) in Devonport has a deep-sea diving suit on display much like that worn by Johnstone on his expedition.

"This is a suit very similar to what Johnno wore and a very similar diving helmet as well," said Sarah Mae Beckett, the BSMC project officer.

The displayed diving suit recently played a role in the resurfacing of Johnstone's story.

"We run a monthly program called Vino and the Visual," Ms Beckett said.

"It's an art class run by local artist Elysium Greene but also a way to get people engaging with the many items hidden in our collection.

"When we focused on the suit it led to learning more about the Bass Strait walk and we found some photos of Johnno. It just struck us what an incredible story it is."

From salvaging gold to exploring Bass Strait

Looking deeper into Johnstone's life revealed that his ground-breaking Bass Strait dive was not even the most amazing, or dangerous, of his achievements.

In 1941, he led a gold salvage dive on RMS Niagara which had been sunk in 134 metres of water off Whangarei in New Zealand.

"He salvaged 555 gold ingots from that great depth, valued then at 2.7 million pounds," Ms Beckett said.

Johnstone was born in Liverpool in the UK in 1892, the son of a warehouseman.

He worked in shipyards and shipping and then the Royal Navy's salvage section before moving to Melbourne in 1919.

"He was working for the Marine Board in Tasmania, a lot in the Mersey [River], when the cable was damaged in Bass Strait," Ms Beckett said.

"He heard that someone would have to walk on the bottom of Bass Strait so he rang the office and said 'who's going to do that?'

"And they just said 'you are'."

Sarah Mae Beckett in the main gallery at the Bass Strait Maritime Centre. ( ABC Northern Tasmania: Rick Eaves )

Sharks, octopuses, and a 76kg suit

On being commissioned, Johnstone contracted friend Captain Harry Burgess and his wooden ship, Julie Burgess, to act as mothership.

An engineer installed a phone system that ran cables from Johnstone's helmet back to the ship — real-time communication was going to be vital.

In his book, Johnno The Deep Sea Diver, Johnstone wrote:

We set off to a little place called Naracoopa, on King Island. The engineer (in charge of the booster station there) … warned that there were sharks and octopi in these waters, but by then this did not alarm me.

Johnstone and the crew first dragged a hook over the sea bottom to locate the cable.

Over six weeks he followed the cable along the sea floor in his heavy suit.

He was repeatedly lifted 2–3 metres above the sea floor by waves at the surface and dropped back to the seabed.

It was exhausting but sometimes he could cover up to a kilometre in an hour. At other times, currents would keep him from getting anywhere.

Johnstone reported stunning visibility and lamented that he had no camera.

A camera housing was hastily dispatched from Melbourne, allowing Johnstone to successfully photograph the damage to the cable when he located it.

Basslink cable being laid in Bass Strait after the communications and power cable was damaged in March 2018. ( Supplied: Diving Co )

Adopted by fur seals

One day Johnstone was circled all day by a family of seals — parents with a pup.

In his book he wrote that he felt like they had adopted him:

My family were there each day … waiting to welcome me and keep me company … the crew of the boat flung them fish which they leapt for and caught in the air.

Then one day the parents were gone and just the pup, who he had named Percy, remained.

He remained faithful for a time and then he disappeared too:

Percy and I had some special moments together. When I stopped to decompress he would come close enough for me to tickle his nose with a finger. Some time after … I was strolling along a beach on the island (King) and found the body of a young seal washed up … it had a deep gash in its side and half its head was gone. Was it Percy? I couldn't be sure, but it looked very much like my little playmate.

Johnstone eventually retired in Victoria and died in a nursing home in Frankston in 1976.