Olivia Berry and her friend Hayley Cox were out paddleboarding at sunset off Scarborough in south-east Queensland's Moreton Bay when they noticed an unusual "neon-blue" ripple in the water.
"We were heading back as dusk fell," Ms Berry said.
"We noticed the lights started glimmering, and they got more and more illuminated as the sun went down.
"It was really bright, almost like a neon-bright blue, and it rippled with the water."
The 24-year-old has lived in the area for more than 18 years but had not seen the phenomenon before in the Moreton Bay region.
"It is pretty rare from what I understand," Ms Berry said.
"I think a combination of the right wind and tide and also conditions in the water."
Marine ecologist Kylie Pitt said the iridescent blue phenomenon was called bioluminescence, caused by the zooplankton marine species Noctiluca scintillans.
"When [the plankton's] food is really abundant, that means that it can really grow and reproduce, and it can become abundant itself," she said.
"When it forms these dense abundances, it can form these bioluminescent displays."
She said when disturbed, it caused a "chemical reaction" and emitted light.
Professor Pitt said the blooms tended to be short-lived, lasting only for a few days.
Resident Denise Cooper's dogs created big splashes of fluorescent blue water while playing at Queens Beach North at Redcliffe on Wednesday evening.
Ms Cooper was taking her dogs out for their usual walk when she encountered the bioluminescence.
Phenomenon now common further south as climate changes
Fisherman Shane Roberts has been on the water in the area for more than 40 years and said the phenomena happened at least yearly in Moreton Bay.
"We don't see it a lot close to shore in the bay itself," he said.
"In the bay, it's out a little bit further from the land, so unless you're in a boat, really, you don't see it all that much from the land."
He said the "sea sparkle" lit up the back of his boat "like a big flame" on his last adventure while he was in the eastern part of the bay on Wednesday night.
"It's magical," Mr Roberts said.
He said other fisherman reported sightings in April but said it was "nowhere near as thick and as bright and brilliant" as it was this week.
Professor Pitt said it was a well-known phenomenon but was only occasionally seen in large blooms around the Moreton Bay region.
She said bioluminescence was very common in coastal waters in New South Wales and only recently had become common in Tasmania.
Professor Pitt said there was growing concern that the blooms were occurring more regularly.
"They have not historically been found that far south, but now they occur down there very commonly," she said.
But Professor Pitt said seeing blooms was not necessarily a direct indication of a problem, as there could be multiple reasons for high levels of nutrients in the water.
"One is a completely natural phenomenon, which is an upwelling of the deep waters from offshore," she said.
"The other way that you can get increased nutrients is through human influence.
Is it safe to swim in?
Professor Pitt said bioluminescence was safe for humans.
"I've swum around blooms before, and it's really quite good fun," she said.
Mr Roberts said the best time to see it was when the water was "really glassy and calm".
However, Professor Pitt warned the aftermath could cause harm to marine life.
"When they are really highly concentrated, and when they all die at the same time, the decomposition of them actually sucks up a lot of the oxygen in the water column," she said.
"Sometimes that loss of oxygen can actually be detrimental and cause animals around it to die.
"So occasionally, they're associated with fish kills and deaths of other organisms in the marine environment."
