Organisers of the Australia Day Aquathon are considering moving the event's swimming leg 400 metres up the beach in response to a sewer leak in Wollongong Harbour.
Wollongong City Council closed the harbour to swimmers at the weekend after a foul-smelling liquid was detected leaking from a sandstone wall near the jetty, at the harbour's southern-most side.
With the aquathon expected to draw up to 2000 swimmers into the harbour six days from now, event director Rob Battocchio said he was confident the leak would be contained in time, and any health threat ruled out.
He added council had closed the site at the weekend only as a precautionary measure, and said no specific health risk had been identified.
"We hope that once the authorities can delve a little bit deeper, that measure [the closure] will be addressed," he said.
If the leak is still considered a potential hazard come race day, event organisers will take advice from environment and water authorities, and council.
In the meantime, contingencies are being considered.
Mr Battocchio said the "most immediate" of these would involve moving the start of the swim leg 300-400 metres up the beach.
"The aquathon swim is well away from southern area wall and we plan to move the swim further away to northern aspect 'little harbour' as a precautionary measure," he said.
He added the safety of participants was "the first and foremost priority" of event organisers.
The council said authorities could not confirm the cause of the leak, so were being cautious.