One of Wollongong's most popular children's swimming spots has been closed this week, after a diesel spill from a charter boat contaminated the water in Wollongong Harbour on Sunday.
However, Wollongong City Council said the Aquathon and other Australia Day activities were "unlikely" to be affected, despite ongoing safety concerns at Brighton Beach.
"We understand the NSW Roads and Maritime Service, who are the agency responsible for the health of the Harbour, are aware of the incident and worked with NSW Fire and Rescue to manage the spill over the weekend," the council said.
"However, Belmore Basin is a popular place for swimming and small children often play at the water's edge.
"Contact with hydrocarbons can cause skin irritation and that is why we've placed signage around the Harbour area warning people to stay out of the water."
The council says it will continue to monitor the water conditions and expects warmer weather and sunshine to help the diesel to break down.
"We're also using a jet ski in the Harbour to disperse the diesel and encourage it to evaporate," the council said.
"Daily inspections are being made and today's review has shown the amount of diesel has reduced."
Water contamination has plagued the Australia Day Aquathon in the past, with a sewage leak raising concerns in 2014.
Throughout that January, people reported that they had broken out in rashes after swimming in the harbour.
Sewage had been leaking from a pipe near the Fisherman's Co-op building, and tests conducted by Sydney Water found high levels of ammonia, faecal coliforms and enterococci in water close to the leak.
The water was cleared for the event to go ahead,
Since its inception, more than 16,000 participants have taken part in the Aquathon, which is now in its 21st year.
It will again feature four swim and runs; which can be completed as an individual or relay of two (a swimmer and runner).
There's also the 4km scenic Blue Mile "Warrigal walk-run-roll".
This year all fundraising and donations would be directed to the charity's bushfire appeal - providing clothing, counselling, food and financial assistance to affected communities.
To enter or for details visit www.aquathon.com.au