A relaxing stroll along Sydney Harbour could have turned deadly for one man and his dog after a brush with one of Australia’s most venomous sea creatures.

Jesse Donnison and his dog Otto were walking along Blackwattle Bay in Glebe when he spotted an expensive dog toy floating in the water.

But Mr Donnison got more than he bargained for when he picked up the ball, thinking he was going to score a free toy for his dog.

“I just thought it had algae or something like that on it but then a tentacle popped out,” he told NCA NewsWire.

He initially “freaked” and dropped the ball, however, had a heart-stopping moment when Otto chased after the toy and the creature.

“As soon as that tentacle came out, I knew there was only one creature it could be,” he said of the blue-ringed octopus.

“I dropped it pretty quickly, more than anything I was worried about the dog. Otto tried to get onto the ball immediately.

“Lucky I even looked and didn’t just chuck it for the dog to go and grab.”

Blue-ringed octopuses are among the world’s most venomous marine animals, carrying enough venom to kill 26 adult humans within minutes.

Due to their size, their bites are tiny and often painless, with their venom capable of causing respiratory arrest, heart failure, paralysis, blindness and eventually death from suffocation.

“I wasn’t so much scared as I was startled. I was expecting sea weed, so it was a bit of a surprise,” Mt Donnison said.

He said the octopus started off at a green seaweed color but quickly began to show its blue rings.

Though they are venomous, the octopuses are relatively docile, only showing their bright blue rings and becoming dangerous to humans when they believe they’re being threatened.

Just weeks ago a woman was lucky to escape with her life after she was bitten multiple times by the creature.

The woman, in her 30s, was bitten on her stomach on a Thursday afternoon at about 2.45pm at Chinamans Beach in Mosman.

“This woman was swimming and picked up a shell. It contained a small blue-ringed octopus which fell out and bit her twice on the stomach,” NSW Ambulance Inspector Christian Holmes said.

“The patient was experiencing some abdominal pain around the bite site, so paramedics applied pressure and a cold compress before taking her to hospital to be monitored and treated for further symptoms.”

Mr Donnison said that woman was front of mind when dealing with the creature.

“I stayed right away after hearing what happened to her. I knew blue-ringed octopus were around but not at Black Wattle near Glebe,” he said.

Blue-ringed octopus are found across the east coast of Australia and throughout Sydney Harbor.