Charles Winn’s 1878 painting of St Helena Island.

Historians have identified a rare treasure of Brisbane’s past: the only painting of St Helena Island as a working prison, as painted by an inmate.

The watercolour has been donated to the Museum of Brisbane by its owner, Trisha Anderson, and will take pride of place in an exhibition about Moreton Bay opening in February 2014.

“It’s just an amazing story of coincidence and fate,” Ms Anderson said.

The Hendra resident’s grandfather, William Gall, was comptroller of Queensland prisons in the late 19th and early 20th centuries.

“One of his jobs was to go across to St Helena and check on the prison there... the painting was presented to my grandfather when he retired in the early 1920s,” she said.

“Since I moved to this smaller house, it hasn’t been on a wall, so I had it wrapped up in plastic and towels and hidden in a wardrobe.”

Ms Anderson said she knew the painting was precious, but was never sure what to do with it.

Then she attended a talk given by Ron Kelly, curator of the upcoming MoB exhibition The Many Lives of Moreton Bay.

“I went and spoke to him afterwards, saying ‘I’ve got something you may be interested in,’” she said.

Mr Kelly, MoB director Peter Denham and a St Helena national park officer then visited her home to inspect the landscape, and realised very quickly they had a one-of-a-kind find on their hands.

“It was so funny watching them all, as they all had magnifying glasses and they were all saying “Oooh, look at that!” she laughed.

“They were even trying to work out which way the wind was blowing by the smoke coming out of the chimney of the sugar mill.”

Mr Denham said it was one of the most exciting finds of his ten years in the role.

“She’s really added to our knowledge of the city, and it’s a great gift to everyone.”

He said historian Lauren Penny was able to identify that it was painted in 1878 by 33 year old inmate Charles Winn.

“He was a draughtsman, architect and artist... obviously he fell on hard times and got convicted for vagrancy and sentenced to hard labour,” he said.

“It seems though that he painted this picture while he was a prisoner, which doesn’t sound too much like hard labour, so I’m glad that it wasn’t all bad news.”

The landscape reveals a small bridge from St Helena’s famous long jetty to the jail which no one knew existed.

The sparse vegetation on the island’s rolling hills in the painting also shows how much of the island had been cultivated for use.

Ms Robinson hoped her donation might spur others on to take a closer look at their own heirlooms.

“Now that we know the history of this – just think how much must be shoved in a trailer and taken to the dump, all these wonderful treasures.”