Canterbury Regional Council says the grading of water as unsuitable for swimming at popular Christchurch beaches has been made to protect people's health.
Several beaches in Whakaraupō/Lyttelton Harbour have been deemed unsuitable due to high levels of contaminants in the water.
Council water quality team leader Shirley Hayward said the water quality in these areas has consistently failed to meet Ministry of Health guidelines.
The grading was based on the past five years of data.
"But in particular last summer there was a number of rainfall events that probably affected the water quality, resulting in increased runoff of faecal bacteria," she said.
"The difficulty we have is we can't always identify the causes of faecal contamination."
Corsair Bay, Cass Bay, Rāpaki Bay, Sandy Bay, Diamond Harbour Beach, Purau Beach and Akaroa Beach have all been deemed unsafe.
Hayward understood it was disappointing for the public, but after some high readings, it was better to be safe than sick.
"There's just a risk that there could be pathogens present. It doesn't mean they will be present, but it means that some people may experience stomach upsets or respiratory problems or ear infections, for example, if they are in contact with contaminated water."
Te Whatu Ora Waitaha last week issued a health warning for Cass Bay, one of the affected beaches, when high levels of faecal bacteria were found in water samples.
"Water quality at Cass Bay is not considered suitable for recreational uses including swimming because of the risk to human health from the bacteria and other pathogens," said Dr Ramon Pink, Medical Officer of Health.
This included gathering kaimoana.
Eating shellfish from these sites should be avoided. If any were eaten, the gut and liver should be removed and the food washed in clean water before cooking, he said.
"Anyone that experiences gastrointestinal illness should contact their GP in the first instance or call Healthline."
Even boating in these areas may still carry a risk of illness, Hayward noted.
"Boating activities, if you're not going to fall out or if you're less likely to fall out, they are less of a risk. But sensible practices such as washing hands before eating is advised."
The council was continuing to investigate contamination causes and routinely testing the water every week.
But if people wanted to cool down on a hot day, there were still plenty of options. Hayward recommended coastal beaches in the city's east - all of which were deemed swimmable.
"There are a number of our coastal sites that do have good water quality and pretty consistently have good water quality."
This includes Waimairi Beach, North New Brighton Beach, New Brighton Beach, Southshore Beach, Sumner Beach and Taylors Mistake.
Water quality can be found on Land Air Water Aotearoa's site.
Other regions also affected
Earlier in December a number of Auckland beaches were marked unsafe to swim at, following heavy bouts of rainfall.
Further south, in the past week sewage leaks have closed New Plymouth's Ōākura Beach and Whakatāne's Ōhope Beach, each for a day.
And Northern Region surf life savers have warned holidaymakers predicted extreme weather could create further hazards at beaches across the North Island.
Strong wind warnings have been issued for Auckland, Northland, Bay of Plenty, Waikato and Coromandel, with torrential rain, gales and surging swells expected throughout the remainder of the week.
Surf Life Saving Northern Region Lifesaving operations manager James Lea said beachgoers should take care as conditions change.
"Weather and surf conditions across the country have changed over the past few days as the tropically charged storm approaches and many beaches are already experiencing moderate to gale force winds and choppy surf."
If lifeguards did not put up red and yellow flags and instead displayed 'no swimming' signs and red flags, Lea asked beachgoers to respect that the water was too dangerous for recreational activities.
"Strong surf will create strong currents. There will be a lot of water moving that can easily knock you off your feet, so keep an eye on close children, making sure to stay well away from the water," he said.
Eastern coastlines from Northland to Coromandel could expect heavy swells that may not ease until Friday, so surfers in particular were being urged to not overestimate their abilities or underestimate the environment.
"We also ask that storm-spectators and rock-fishers take extra care if spending any time on the rocks this week. Large swells can be unpredictable and easily knock you into dangerous waters," Lea added.
"Before you head out, monitor the Safeswim site for updates and please only choose patrolled beaches for swimming and other water-related activities this week."
RNZ
