An oily looking slick that has appeared on the water surface at Belongil Creek in recent days is marine blue green algae or Trichodesmium.
Staff investigated the slick this week and took samples which were sent off for testing.
Chloe Dowsett, Council's Biodiversity and Sustainability Coordinator, said while the film on the top of the water looks terrible, it is a natural event and important for the marine environment.
"Marine blue green algae is naturally occurring and travels down the coast with the east Australian current during the summer months," Ms Dowsett said.
"It sometimes gets trapped in our coastal creeks and because of its appearance people often think it is an oil spill or something toxic," she said.
"This algae is often slimy and smells and can vary in colour from red, brown, green or cream.
People should avoid swimming and not eat shellfish in areas that are affected by the algae.
