Concerns had been raised that a disease or contaminant could be involved after many dead or disabled gulls were found near the Shell Cove Marina.

Testing had been unable to find a cause of death, Shellharbour City Council said, but would continue with toxicology tests in coming weeks.

But Betty Spilsted, who has been helping rescue seabirds, particularly on the shores of Lake Illawarra, for more than 20 years, believes the answer is much simpler: not enough food.

Ms Spilsted said a wetter-than-usual summer had prompted the birds to squeeze in an extra breeding cycle for the year - and the overpopulation led to malnourishment.

"With this ridiculous weather that's happening - let's talk about climate change, as everybody is - with this ridiculous weather, they go a bit silly like ducks and they can just breed constantly, constantly," she said.

"Even this morning I went down [to the foreshore] and it's just a sea of juvenile babies and they're just all running around crying - they're starving.

"There's just too many birds and not enough food."

Concern about a water-borne cause of the deaths was heightened given the marina's popularity as a swimming spot for kids.

Ms Spilstead said some people had suspected the culprit was botulism, but after the extra breeding "season" there were simply too many mouths to feed.

She said all the sick or morbidly weak gulls she found were significantly underweight.

"It is not botulism, it is birds overbreeding and not enough food so that juveniles are starving," she said.

"If [you] weigh them ... they're just skin and bones and that's all the problem was, not contaminated water."

Ms Spilsted said there had been "probably a dozen" gulls dead at Berkeley over Christmas and she was still seeing young birds in trouble.

"I go down to Berkeley every morning ... and I often pick them up. I've got one here at the moment in care that's starving, absolutely starving. He couldn't stand up.

"What I normally do I just walk amongst them and if they fly away or they can't run away, I just pick them up and bring them home, feed them up for a couple of days, make sure they're OK and take them back down."

This was no reason to feed the birds at the foreshore, she said, warning people off chucking food waste out for the birds, who would not handle human food well.

Shellharbour City Council last week said the gull deaths were not unique to Shell Cove and had also noticed in other places across the cities of Shellharbour and Wollongong.