Mandurah Mail reader Cherilyn Corker shared her photos of a surprise visitor in the Mandurah Estuary.
Ms Corker said she was "very surprised" to see a Portuguese Man o'War sailing up the estuary into Soldiers Cove on Monday.
"As it came close to the bank local seabird rescuer Craig Lester and I were able to see the creature clearly moving its body, and even collapsing its air filled sail at one point," she said.
"Luckily no-one is likely to be swimming in the estuary at this time of the year."
The Mandurah find is unusual given the fact Portuguese Man o'War rarely travel alone.
According to experts Portuguese Man o'War enjoy warmer waters and are driven to bays and beaches by strong winds.
Portuguese Man O'War are poisonous with up to 10,000 stings reported in Australia each year, usually on the east coast.
The stings do cause severe pain for humans and can result in welts and an allergic reaction.