Swimming after rain: what our new water-quality model really shows

Recent media coverage has suggested large parts of Whakaraupō Lyttelton Harbour – and almost a third of Waitaha Canterbury’s monitored swimming spots – are unsafe for swimming following recent rainfall. 

We want to clarify what our water-quality monitoring is actually telling us, and how people can best use this information to make safe decisions. 

A new, more accurate way of assessing swimming safety

We have recently released a predictive water-quality model for popular swimming spots across Waitaha. 

This model is built using more than 11 years of water-quality sampling data, combined with real-time environmental conditions such as rainfall, river flows, tides, and weather patterns. Using this information, the model estimates hour-by-hour concentrations of enterococci bacteria, which are used to assess the risk of illness from swimming. 

Water and Land Science Manager Dr Elaine Moriarty said this approach provides a much more accurate picture of current swimming conditions than traditional methods alone, such as: 

• Long-term site grades, or 
• Weekly water-sampling results can quickly become outdated after weather changes. 

“Our new predictive water-quality model gives a far more accurate picture of swimming water quality than a single sample or long-term grade,” Dr Moriarty said.  

“Under dry conditions, around 80 percent of Canterbury’s monitored swimming spots are suitable for swimming. It’s rainfall – not an ongoing issue – that temporarily increases risk.”

What the model shows across Waitaha 

Under dry weather conditions, 77 per cent of monitored swimming spots across Waitaha are suitable for swimming. 

However, water quality can decline rapidly after heavy rainfall, particularly in areas affected by stormwater runoff, rivers, or catchments with urban or rural inputs. This is why we consistently advise people to avoid swimming for at least 48 hours after heavy rain, especially near river mouths, stormwater outlets, and harbours. 

Importantly, this does not mean that swimming spots are permanently “off limits”. In many cases, water quality improves quickly once conditions settle.

Understanding Whakaraupō Lyttelton Harbour results 

Lyttelton Harbour includes a range of swimming locations with very different catchments and influences. Conditions can vary significantly from one bay to another, and even hour to hour. 

The predictive model allows us to reflect this variability far more accurately than a single blanket assessment. While some locations may show elevated risk immediately after rain, many sites return to suitable swimming conditions once rainfall and runoff effects subside.