More than 9800 Australians are expected to die from melanoma in the next five years, with the number being drawn in sand at Bondi Beach on a sweltering day.

Bondi beachgoers escaping a Sydney heatwave have been reminded there's a far more immediate danger to their lives than what may be lurking beneath the waves.

As the mercury hit 35C in Sydney before 8am, swimmers and surfers were confronted with the number 9883 scrawled in the sand, representing how many Australians will die from melanoma by 2021.

Almost 90,000 Australians will be diagnosed with melanoma within the same time frame, something that Australia Cancer Institute NSW spokeswoman Alecia Brooks says is 95 per cent preventable.

"I think there is definitely a little bit of complacency and thinking that skin cancer or melanoma is something that's not as serious, that they don't need to worry about it, but it clearly is by looking at the number in the sand today," Ms Brooks told AAP on Wednesday.

While Australia is no longer the world leader in melanoma rates, having recently given up top spot to New Zealand, skin cancer is still on the rise, Ms Brooks said.

"Numbers are going slightly up, but that's because we've got a slightly ageing population that grew up without the sun protection message," she said.

Ms Brooks says it was important to be proactive in preventing skin cancer, rather than reacting to it, which weighs on the public health system.

"It definitely does put a burden onto the health system and that's why campaigns like this are so important because we know they're effective and we're trying to prevent people from having to go into the health system in the first place," she said.