With each day growing warmer, you might be inching that step closer towards your first cold plunge of the summer.

But there are a few things to consider before you dive in.

For one thing, our beaches aren't really ever 'safe' or 'unsafe' to swim in.

Safeswim's independent Head of Science Martin Neale explains.

It's not 'safe' or 'not safe' to swim

Safeswim is a water quality programme offering information on swimming conditions and safety hazards around beaches in New Zealand.

It's an important tool to consider before you head out for a swim, but it's equally important to remember the programme is 'risk-based', Neale says.

Safeswim's Holly Foreman and Martin Neale. Photo: RNZ / Samuel Robinson

"It's not 'safe' or 'not safe', the only place you'll see the word 'safe' is in the name Safeswim ... we're just telling people it's low-risk today or it's high-risk today. We have no mandate to stop people swimming, we're giving people the best information we can and letting people make that choice."

There's always a little bit of poop in the water

Safeswim collects samples of about 100ml of water from local beaches once a fortnight.

Those samples are then tested for something called faecal indicator bacteria - the bacteria that comes from the faeces of warm-blooded animals like birds, dogs, cows, and yes, humans.

The results of those samples can indicate whether water is low or high risk.

Unfortunately, there's no such thing as 'no risk'.

"The bacteria is always there, generally in small concentrations," Neale says.

How does poop get into the water anyway?

Safeswim and other water quality programmes test for faecal indicator bacteria, largely because it's easier to test for than most other bugs and viruses.

"They're bacteria that we find in the stomachs and intestines of warm-blooded animals and there can be up to 15 million of these things per gram of faeces from an animal. They indicate the presence of fecal contamination in water, and they're chosen as indicators because they don't live very long in the environment."

Animals do their business in various places, and when it rains, the faeces will often get washed off the land and straight into beaches, rivers and streams.

Neale says the sources of contamination at beaches are different - that's why you'll see some beaches with more frequently poorer water quality than others.

"We know there are lots of places where birds gather for example.

"Big, natural flocks of birds do their business and it has an impact on water quality ... we have places where there's rural activities, in those types of places where streams are not fenced, the cows and sheep do their business and it flows downstream into a beach.

"In and around a lot of urban areas, we get runoff from concrete but we also do have known issues with the wastewater network in places. Obviously the human side of things is a little more complicated, we have wastewater networks, and perhaps not by design, but some are leaky to some degree."

How are some beaches low risk one day, and high risk the next?

The state of a beach will change over time due to two main reasons - discharge and dilution.

"If you get a discharge to a beach, over a couple of tidal cycles, there's a huge amount of clean ocean water coming in that dilutes the contamination to lower levels. We can see in open coast beaches where you get big tidal flashing, contamination can dilute and drop below guidelines for swimming quite quickly.

"Whereas some of the inner harbour areas where there's not so much flashing, sometimes the contamination can get stuck there for longer, and it requires more tidal cycles. That's one of the big ways that water quality changes over time - we might get a discharge to a beach that increases the concentration of bacteria in the water above a certain level, and then we get dilution through the tidal cycles that reduces that."

How is it that neighbouring bays, sharing the same body of water, can have totally different water qualities?

Even if two bays are right next to each other, only the beach with the discharge directed at it will be affected, Neale says.

"So if a beach has a big stormwater pipe discharging to it as many of them do, or a stream coming across the beach, we see that beach would be more susceptible to water quality impacts than, let's say, a beach with no discharges on it."

Tāwharanui Regional Park - View of Anchor Bay from Flat Rock looking west. One of the cleanest beaches to swim in Auckland. Photo: Auckland Council website

Is water self-cleaning?

Luckily faecal indicator bacteria can only survive the warmth.

That means the second they get into salt water, they die off pretty quickly.

"The bacteria live in anaerobic conditions so once they get out, they don't live for very long, maybe a few days. And in addition to the salt water action, another thing that is known to be impactful on bacteria is UV radiation.

"There's a fair bit of science around the world that says on sunny days, the water quality will tend to improve quicker if you've had a contamination. If it's sunny, you might see greater die off of bacteria because they're quite vulnerable to UV radiation."

Can that little sample of water really help determine the risks?

Sampling alone isn't enough to indicate water quality - it's just one element of a larger modelling and forecasting approach at Safeswim, Neale explains.

"Samples are not done every day, the reasons for that are two-fold. One, if we take a sample from a beach to test it, we don't get that result back for two days ... that means whenever we use monitoring results in that way, we're always telling people information that could potentially be out of date.

"The other thing is ... if you take one sample a week or fortnight, which is the norm for most monitoring programmes, you've got a one in seven chance of actually catching contamination if it occurs ... we do a lot of sampling and monitoring at all of the beaches, and what we use the sampling data for is to test and refine the predictive models.

"We'll take a small sample and compare what the model says at that point in time and see if it's correct, see how accurate our models are. Every sample we take gets tested for the fecal indicator bacteria, and if that result then comes back high or above the swimming guideline, we get a message from the lab quite quickly to say, 'This sample is high'.

"We can then look at it [and] investigate if it's an unusually high sample and the source, whether it's a human source or from dogs or birds or cows. The reason we want to know that is the management of the water quality problem will be different."

Safeswim collects data around the cleanest and dirtiest beaches around the Auckland region.

These were the ones to avoid, and the ones to dive into, according to the 2022/2023 rankings.

The five dirtiest Auckland beaches

1. Fosters Bay, Waitākere Ranges

2. Green Bay, Whau

3. Lynfield Cove, Whau

4. Meola Reef, Albert-Eden

5. Wood Bay, Waitākere Ranges

The five cleanest Auckland beaches

1. Tāwharanui, Rodney

2. Anchor Bay, Rodney

3. Devonport. Devonport-Takapuna

4. Goat Island, Rodney

5. Karekare Beach, Waitākere Ranges