Thirroul man Peter Armstrong noticed the dolphin early on Tuesday afternoon, first reaching for his binoculars then using a drone to identify what was bobbing atop the water about 200m off the beach.

It was a large adult dolphin, dead and bloating after getting its tail caught in a shark net. Unable to escape, that's where it died.

"It was a fair way out - it's directly in line with the drum lines that are out there," Mr Armstrong, 51, told the Mercury.

"I'm there surfing all the time and in the water most days - dolphins are coming past all the time.

"It's a bit of a shock I guess. They [the nets] do really catch mostly species they're not meant to catch. It's a shame."

The large dolphin had been there for some time, Mr Armstrong said.

"It looked like it looked bloated and the [snout] looked like it had just started to decompose because it was in the sun, the body was a little bit bleached as well.

"At first I thought it was a tiger shark because it was a little bit mottled. I couldn't see the tail because that was what was caught in the net ... but then I got the drone quite low."

Environmentalists regularly call for shark nets to be removed as NSW Department of Primary Industries statistics show just 10 per cent of all animals caught were the target species - and more than half the "bycatch" animals are found dead.

Mr Armstrong, a northern suburbs real estate agent, said his opinion on shark nets was clear.

"It's ridiculous - absolutely ridiculous," he said.

"It's been known for years and years how dangerous they are and what they do to all these marine animals.

"They should go - it's crazy.

"I'm dead against them - I'm even against the drum lines too.

"I use the ocean on most days - we see sharks, we see lots of life and it's part of the whole ecosystem."

The NSW Department of Primary Industries says shark nets off Sydney are checked at least every 72 hours.

Shark nets are installed at five Wollongong beaches - Thirroul, Coledale, Austinmer, North Wollongong and Wollongong city beach.

There are also 16 SMART drum lines on beaches across the Wollongong LGA, and 12 at four Shellharbour beaches.

Statistics from the 2021-22 summer showed that on the Illawarra coast 12 of the 128 animals caught were the target species white, bull or tiger sharks.

The rest were other species including 15 Bronze whaler sharks, 13 of which were dead, 11 hammerhead sharks (all dead), 43 Southern Eagle rays, two dead Grey nurse sharks and two dead green turtles.

The largest sharks were a 4m Tiger shark found dead in the net at Wattamolla, two 3.6m Tiger sharks found dead in the net at Garie Beach, a 3.3m Tiger shark found dead at Austinmer.

The Coledale beach shark net went missing in March 2022 and was not found.