The concrete structure at the back of his house on Medusa Street is around three metres long and a couple of metres wide, with a round roof and a square entrance. It sits a couple of feet below the ground. And there's no doubt about it - it's solid.

"Dad thought one day he'd knock it down - he took a hammer to it, I think he only managed to make a small dent," Wild said.

While it's not unheard of, it's rare to come across a house in Sydney with an air raid shelter in the garden.

Now, the 1920s four-bedroom home is on the market.

While Mosman Council doesn't keep a record of how many of this shelters have survived, Mosman Local Studies Librarian, Mary Lou Byrne, said Mosman may have had more shelters air raid shelters built around World War II than other parts of Sydney because of its proximity to Sydney Harbour.

A representative from the Australian War Memorial said the number of shelters installed - and remaining - in Sydney was difficult to ascertain, as most people bought air raid shelter kits and constructed them themselves.

From time to time properties with shelters come onto the market - a home in Greenwich with one was listed in 2013. Another property at North Narrabeen sold the same year, advertised as having a bomb shelter in the basement.

For the real estate agent selling the property on Medusa Street, it's also been a one-off occurrence.

"This is the first one I've seen in and around the Lower North Shore," Brad Rogan said. "You don't really see them in people's backyard."

Wild says the neighbourhood in general has changed a lot since he was a child.

"When we moved here it was a very ordinary little street, now it's got all these big homes. When I was kid we still had a cart with horses dragging ice around."

Wild said the shelter's main appeal has been to children. "I have three sisters and we used to make a cubby house out of it. Our little grandsons play in there - it's a child's thing now."

It's also come in handy during the Sydney to Hobart race when he and his wife Jenifer stand on top of the shelter to catch a view of the yachts sailing by.

"But it doesn't do a lot these days," he says. "I've never been banished there by my wife or anything."