Heavy rain is on the way for Auckland and parts of the North Island on Wednesday while some of the city’s beaches remain unswimmable following faecal bacteria contamination from previous rainfall.

A warning is in place for the Coromandel Peninsula, which could have a deluge of up to 140mm of rain about the ranges, causing streams and rivers to “rise rapidly”, MetService said, with surface flooding and slips possible.

While the Super City can expect sweltering temperatures, MetService issued heavy rain watches for Auckland, the Bay of Plenty, Rotorua, Gisborne and Great Barrier Island from about 10am Wednesday.

Rain in Auckland will develop in the morning and could become heavy at times north of the Harbour Bridge, MetService said. The day’s high is forecast for 23C.

In the Bay of Plenty, west of Whakatane and south of Rotorua, there could be periods of heavy rain. Tauranga could reach 21C and Rotorua can expect to sit at 19C.

Gisborne, north of Tolaga Bay, is included in the heavy rain watch. The wet weather is brought by a slow-moving low-pressure system to the north of the country.

Humid conditions had been forecast for the week with the warm air mass moving across the North Island. Temperatures this week were expected to be above average for this time of the year.

MetService meteorologist Alwyn Bakker said the exact timings for the downpours could be hard to predict. Unlike cold southerly air masses, Bakker said, tropical air from the north could be changeable.

The city’s water quality monitoring website, Safeswim, has warned the public against swimming at the majority of Auckland’s beaches due to the increased risk of illness.

After recent rainstorms, most of the city’s beaches are marked as high-risk on the Safeswim map due to high faecal bacteria contamination.

Contaminated water can contain harmful bacteria, viruses, and protozoa that can cause illnesses such as gastroenteritis, respiratory illness, ear and eye infections, and skin infections.

In addition to the general stormwater contamination, there have also been multiple wastewater overflows, which pose an even greater public health risk.

Watercare is working on multiple projects to reduce wastewater overflows and improve water quality in Auckland.