Stretches of Waikiki’s white sands and blue waters were deserted on Tuesday after officials warned that heavy rains had triggered a sewage spill of half a million gallons (1.9m liters) near Hawaii’s best-known tourist district.

Dozens of tourists still waded into the water, and young parents carried their toddlers into the ocean, ignoring the warning signs about unsafe water.

Carmen Antaky went swimming in Waikiki with her friend Sloan Hill-Lindsay on Tuesday. “I guess a lot of people are freaked out about it,” Antaky said. “We still jumped in.”

They said they went in the water every day and were willing to take their chances. “We’re also from LA where it’s pretty polluted anyways,” Hill-Lindsay said.

The heavy rains overwhelmed the sewerage system on Monday morning, causing 500,000 gallons of wastewater to spew from manholes, according to Lori Kahikina, Honolulu’s director of environmental services. “Now’s not the time to go swimming,” she said on Monday.

Lifeguards gave verbal warnings that the water was polluted, but without enforcement powers all they could do was warn visitors, officials said.

Peter Parhar, from Vancouver, who was also on Waikiki beach on Tuesday, said the infrastructure should be improved and he was concerned about the city’s communication of the issue.

“More should be done to inform the beach patrons of the risks of being in the water,” Parhar said. “For the tourist segment that isn’t English-speaking, they have no idea what’s going on and they’re carrying little infants into the water.”

The city is advising people to avoid a four-mile (6.4km) stretch of waterfront. Sewage came out of manholes at Ala Moana beach park, on a street fronting a shopping mall and a pumping station. The park is closed and Honolulu police were keeping people away on Tuesday.

Waikiki was not the only area of Oahu affected by an overflow of wastewater. The Department of Environmental Services said Kailua, Kaneohe and Kalanianaole Highway in Aina Haina also experienced overflowing sewerage systems. The Department of Health issued warnings for people to stay out of the water in all affected areas, including Kaneohe and Mamala bays.

Shayne Enright, a spokeswoman for the city’s Department of Emergency Services, cautioned that the ocean was dangerous. “We don’t know right now what is in the water,” she said. “You could get a serious infection, get extremely sick or even worse.”

Kahikina said stormwater had entered the sewerage system as leaves and debris clogged storm drains. Some witnesses reported people illegally opened manhole covers to let the stormwater drain into the system, even though sewerage pipes and pumps aren’t designed to handle that volume of liquid.

In 2006 the city temporarily closed Waikiki’s beaches after 48m gallons of raw sewage poured into the Ala Wai canal bordering the area’s hotels and condominiums. That spill occurred after a sewerage line ruptured following weeks of heavy rain, forcing the city to divert wastewater into the canal.

The entire state remained under a flash flood watch again on Tuesday, with more rain expected. A new tropical storm, Ignacio, formed east of the Hawaiian islands and is forecast to become a hurricane by Thursday, according to Central Pacific Hurricane Center meteorologist Chevy Chevalier. “It’s an above-average year already and we’re still just in August.”

There have been 17 storms so far this year in the Pacific, 12 of which reached hurricane status, according to the National Weather Service. Six of those storms reached super-typhoon status, which means they had maximum sustained winds of more than 150 miles an hour.

Ignacio’s current path could take it near or over the islands, Chevalier said.

“The reason for the forecast of an above average tropical season in the Pacific this year is El Niño conditions,” he said in an email. “El Niño typically brings this area above-normal sea surface temperatures and less vertical wind shear, both of which normally lead to tropical cyclone intensification.”