The risk of illness from swimming is associated with levels of micro-organisms in the water, called Faecal Indicator Bacteria (FIB). Faecal indicator bacteria are bacteria that come from the gut of warm-blooded animals (including people). They indicate the presence of faeces (poo) and are associated with the presence of disease-causing pathogens. The two common bacteria that are measured are E. coli, in freshwater, and Enterococci, in seawater.
Guideline values for E.coli (freshwater) and Enterococci (seawater) are specified in the 2003 Microbiological Water Quality Guidelines for Marine and Freshwater Recreational Areas published by the Ministry for the Environment and Ministry of Health and have been selected to keep the risk of illness from swimming at less than 2%. A 2% risk of illness means that, on average, around 1 in 50 people would be likely to become ill after swimming. The swimming guideline is based on the risk of getting sick from being immersed in the water for 10 minutes and putting your head under the water three times.
Green pins are shown when Safeswim’s water quality models predict that levels of faecal indicator bacteria (FIB) at a location meet national guidelines for swimming. Where a green pin is showing, the risk of illness from swimming is estimated to be less than 2%. However, it is important to remember than a green pin means low risk, not no risk. Microbial pathogens may still be present and you may still get sick from swimming.
Some sites on Safeswim have permanent green pins. These are sites where there are no significant sources of faecal contamination and long-term sampling results indicate few (if any) exceedances of the swimming water quality guideline and consistently low FIB levels. Dynamic water quality modelling is not required at these sites.
Red pins are shown when Safeswim modelling predicts that the level of faecal indicator bacteria (FIB) at a location exceeds national guidelines for swimming. Where a red pin is showing, the risk of illness from swimming is estimated to be more than 2%. The actual risk of illness from swimming when a red pin is showing could be much higher than 2% because the concentration of FIB in the water could be much higher than the trigger threshold for a red pin.
A red pin may sometimes be used to indicate the presence of toxic algal blooms, such as cyanobacteria. While the risk of illness cannot be reliably estimated in these cases, swimming is not advised on a precautionary basis. 
Long-term alerts are used at locations where water sampling shows that water quality consistently exceeds national guideline values for swimming and cannot be accurately modelled. Where a permanent red pin (long-term alert) is showing, the risk of illness from swimming is estimated to be more than 10%. 
The black water quality pin on Safeswim indicates that the beach has been affected by a wastewater overflow. While the risk of illness cannot be reliably estimated, Auckland Council and Watercare strongly advise against swimming in locations where a black pin is showing as untreated wastewater is present.