New tracking data has found that bull sharks are travelling further south along Australia's eastern coastline due to a strengthening current.
Bull sharks typically reside in the tropical waters of Queensland but travel south during summer and autumn as the water warms.
NSW Department of Primary Industries (DPI) marine ecologist and shark specialist Amy Smoothey said there was evidence the species had made it as far south as Lakes Entrance in Victoria.
"This is groundbreaking science," she said.
"This is potentially due to the strengthening of the East Australian Current (EAC) and with the availability of their preferred water temperature pushing further south and [staying] around for longer periods of time."
The DPI has tagged more than 250 bull sharks across NSW since 2009, with 87 of the sharks tagged in Sydney Harbour.
Dr Smoothey said the data from DPI's tagging program had significantly expanded their understanding of bull shark behaviour.
"Prior to our tagging work, they were typically only known to come as far south as Wollongong," she said.
Female bull sharks normally give birth in estuaries and river mouths and the juveniles can remain in the river for up to five years.
A juvenile bull shark has been tracked 85 kilometres upstream from the Clarence River mouth to near Copmanhurst, west of Grafton.
According to the DPI, nearly all shark bites in coastal waters were attributed to three species — white sharks, tiger sharks and bull sharks.
Bull sharks were most commonly found in shallow, murky inshore waters.
Fishermen noticing 'more' bull sharks
Fishermen said they anecdotally noticed more bull sharks in harbours and rivers across NSW during summer.
Craig McGill from Fishing Charters Sydney said he had never seen so many in Sydney Harbour before.
"I wouldn't be doing any swimming in the harbour and even on the beaches. I'd be keeping out of those low light hours," he said.
"It is probably as bad as I've ever seen it for sharks out there."
Port Macquarie fisherman Colin Ternaman said he was seeing lots of sharks up the Hastings River, as far inland as Wauchope.
"I've had them pirouetting in the sky, smashing into the mullet and grabbing bream from under the boat," he said.
No evidence of increasing numbers
But Dr Smoothey said there was no evidence to suggest bull shark numbers were increasing.
Instead, she put it down to the distribution of the population moving further south and staying there longer due to the warm ocean current.
"Any temperature above 19 degrees Celsius indicates that bull sharks are likely to be around," she said.
Ocean temperatures are currently reaching as high as 25C off the Victorian coastline.
Dr Smoothey added that heavy rainfall last year could be contributing to where the sharks were being spotted.
"After heavy rainfall, fresh water drains from catchments into rivers," she said.
"This results in murky freshwater plumes that mix with seawater at river mouths, and create sort of a localised water front."
Dr Smoothey said these fronts then caused plankton blooms and an increase in bait fish.
"Where there's bait, there's going to be predators, and that's where bull sharks are likely to occur," she said.
