Mainprize, 27, was on board the Gulf Livestock 1 when it went missing during a typhoon near the coast of Japan three weeks ago.

His sister Sarah Mainprize said despite the time that had elapsed since the ship vanished with 43 crew and more than 5800 cattle on board, she had not given up hope of finding her brother alive.

“There are four life rafts that are missing that automatically deploy once the ship gets to either a certain amount of degrees [tilt] or if it is submerged below four metres,” Mainprize said. “So there are definitely, we know, four life rafts out there somewhere — whether they have people on them or not — which are unaccounted for.”

Ms Mainprize said testimony from two survivors that the remaining 40 men were waiting to climb into the rafts meant they could still be waiting to be rescued. “I’m sure a lot of people would be thinking ‘are we this crazy family that is just holding on with dear life to the last skerrick of hope?’,” she said.

“Based on those key facts, we believe they are waiting to be rescued.”

Call for big tech to help

Ms Mainprize said she was grateful for the assistance of Australian and Japanese authorities to find the men.

More than 50,000 people have signed an online petition begging the Japan Coast Guard to accept Australian government support for the search.

The family is now raising money and hoping the private sector could have the answers.

“Whether that be big tech, satellites, GPS services, anything that may help us to complement the Government’s efforts so far,” Ms Mainprize said.

She said the experience has been “the most truly devastating thing that has ever happened” to her “close-knit family “. “We are so desperate to get him, and also the rest of the crew back, as soon as possible.”

Had been ‘final voyage’

Ms Mainprize said William, a graduate of Charles Sturt University in Bathurst, had been planning to buy a house.

“William is a survival specialist,” she said.

“His day job is taking groups of people through the most remote locations in Tasmania for up to 10 days at a time. “This was going to be his last, proper voyage.”

During the typhoon, William was messaging with his friend Harry Morrison.

Mr Morrison formerly worked alongside William on live export vessels.

“We are in the middle of a typhoon and engine control room is taking on water,” a message to Mr Morrison at 8:36pm said. “Engine is off and we are floating sideways in huge sea.”

Video taken on board the ship by fellow missing Australian Lukas Orda before it disappeared shows the livestock carrier rolling heavily in rough seas.

Ms Mainprize said she spoke to William the night before the ship sunk and had also been concerned about what could happen.

Early the next morning she searched for the ship’s location, but found the top result brought up news it was missing.

“It was a living nightmare to find articles of the fact the ship was missing, and the journey we’ve been on since then,” she said.

The Department of Foreign Affairs and Trade has been contacted for comment.

– ABC / Mollie Gorman