Ocean swimmer Nada Pantle was told she had breached a "no nudity" clause in the club's child-safe policy.
"I almost feel like I've been body-shamed," she told the Australian Broadcasting Corporation (ABC).
But the club says its policy is designed to protect children, and they had received complaints about nudity.
The Terrigal Surf Lifesaving Club, located about an hour's drive north of Sydney, put up signs in its changing rooms advising "no nudity", and directing members to shower in their swimming costumes and change under towels.
Ms Pantle's friend and fellow ocean swimmer Wendy Farley said they first became aware of the new rule earlier this year.
"Probably three months ago, a sign went up… and we all just went 'what?' and kind of ignored it because it seemed crazy," she told the BBC.
"We're not strutting around naked, we're just having a shower and putting on our clothes or having a chat.
"I've been swimming for seven years and I'm 59, I feel more comfortable in my skin than I've ever felt. [When] you're in the change room, you see all sorts of bodies, all ages… no one cares. This feels really prudish and horrible."