mystery propeller sydney harbour
The mystery propeller found in Sydney HarbourPort Authority of NSW

A mystery shipwreck has been discovered at the deepest point of Sydney Harbour by the Port Authority of NSW using a state-of-the-art sonar device dubbed a "fish finder on steroids". The Teledyne Reson Seabat T50-Pt found a massive propeller at a depth of almost 13m. Authorities say they believe it was purposefully dumped – by whom and why, however, remains unknown.

The Port Authority has known there was an object sitting next to a 45m deep hole at Port Jackson for a decade, but no one knew what it was. Using the new sonar device, which the Authority recently bought for A$230,000 (£106,000), the team was able to determine the propeller is at least 4m in diameter and likely weights more than 15 tonnes. It has two protruding blades measuring at least 1.5m.

Sydney Harbour Master Captain Phillip Holliday has been put in charge of finding out where the propeller came from and how it ended up at the bottom of Port Jackson. He told 702 ABC Sydney: "We have obviously been surveying the harbour for many years, but in the area around Blues Point we picked up a pretty strange echo. The more we looked at it, the more we thought we had better take a closer look.

"We sent some divers down and lo and behold there was a full-size ship's propeller down there. It is very close to the deepest part of the harbour; there is a hole to the west of the bridge about 45-metres deep. We are speculating that someone was actually trying to just drop it in the hole and missed."

Brendan Elliot, community engagement manager of the Port Authority, also told ninemsn they believe whoever dumped it was aiming for the 45m hole: "One working theory we have is that the propeller may have been meant for the deep hole, because how else do you explain a ship propeller being where it is?

"There's really no other logical explanation. We don't know at this stage, but that's out best guess. Bear in mind, this thing is of unknown age and origin, so we think it's at least decades old – that's just based on the amount of marine growth that's on the thing."

The Teledyne sonar device produces 50 pings per second, meaning it can map the seafloor with a previously unprecedented level of clarity. As well as showing the mystery propeller, it also revealed other features, including the harbour tunnel.

Elliot said the wreck poses no risk and they are currently waiting for advice on whether it is of heritage significance – which will help them determine whether to leave it where it is or raise it from its watery grave. "We just know that it's there and it's now much better understood thanks to the new sonar equipment we've been using."