A Mandurah family have recounted the "gobsmacking" moment they spotted a boat chasing dolphins near the mouth of the estuary on May 12.
Avid local fisherman and diver John James, his wife Kristy and daughters Chelsea and Kiara were returning from a Sunday afternoon of fishing when they witnessed the "disgusting behaviour".
Mr James said at first they did not notice anything out of the ordinary.
"They were stationery and we could see them taking photos but as we got closer, they started circling the dolphins so we stopped to watch them," he told the Mandurah Mail.
"They were hunting them down and you could see the dolphins were stressed out.
"I reckon there would've been around 15 dolphins and there were bubs in there too because the mothers were trying to shield them."
His daughter Chelsea, 20, said it was terrifying to watch.
"He was doing pretty fast doughies chasing them," she said.
"At one point, he even ran over one.
"We saw everything and I panicked because I thought the dolphin was going to get hurt.
"I actually even screamed a little bit - they were being real idiots and they thought it was funny."
The Biodiversity Conservation Regulations 2018 dictate all vessels must stay more than 100 metres away from dolphins and people in the water must stay at least 50 metres away from them.
Breaching this rule by approaching, herding or chasing dolphins in waterways is illegal and can result in a fine.
Mrs James said they approached the boat to confront the four passengers on board.
"We sped up to them and they saw us and started heading in but we eventually caught up to them and said 'leave the dolphins alone, they are wild animals'," she said.
"They denied it and said they were just taking photos but we were watching them, we saw it all and we got their registration.
"They weren't young, which is what we were even more disgusted about."
Mr James reported the incident to the authorities on Monday, who said they were confident they could track down CCTV footage from the Mandurah Marina and charge the offenders.
"When I spoke with them, they were gobsmacked by what happened and they wanted to get to the bottom of it," he said.
"What they did to a protected animal and to an icon of Mandurah, really p****d me off.
"The only way I could describe it was like he was herding cattle."
Murdoch University's Mandurah Dolphin Research Project coordinator Krista Nicholson said unfortunately it is something that had been reported before.
"We do occasionally hear of, and have observed people, approaching dolphins too close or driving their boat around in circles over the dolphins," she said.
"As the general public has been more informed about the dolphins in our waterways, the desire to interact with them has perhaps also become more prevalent."
Ms Nicholson said the 'rounding up' of the dolphins could disrupt their behaviour.
"If the dolphins are continuously disturbed when resting, it may affect their success in foraging and socialising," she said.
"Also the animals may suffer from stress if continuously harassed, which may result in poor health or even in displacement of individuals - meaning that the animals will stop spending time at a certain location.
"Then there is the obvious trauma from collision or propeller strike."
Instead of approaching wild dolphins, Ms Nicholson advised people who do spot them while on the water to observe from afar.
"If you see dolphins the best thing to do is to slow down or stop and just observe them from afar," she said.
"Often the resident dolphins, especially the juveniles, approach boats and if they do it is just best to stay stationary and enjoy the close encounter on the dolphin's terms.
"Often the dolphins also come and ride the wake or bow wave of a boat. If they do, just keep your speed or slow down gradually to a stop - in this situation it is best to not do any abrupt movements or speed changes as the dolphins know what they are doing and are catching a free ride with you.
"Our waterways are very shallow with less than two metres depth and the dolphins can't escape boats by diving deep. The best approach therefore is to observe dolphins from afar and give them space to move freely."