Microbial assessments of beach water quality

Microbial monitoring of water quality collects information on levels of faecal contamination over time.

Microbial assessment category

The microbial assessment category (MAC) is determined from the 95th percentile of a dataset of at least 100 enterococci data points. The 95th percentile is a useful statistic for summarising the distribution of enterococci data at a swimming site. It embodies elements of both the location of the distribution (how high/low the enterococci counts are) and the scale of the distribution (how variable the enterococci counts are).

The four categories (A to D) relate to levels of risk of illness determined from key epidemiological studies. The National Health and Medical Research Council recommend MAC be calculated from a rolling five year dataset, with at least 20 samples collected each year during the swimming season.


95th percentile enterococci (colony forming units per 100ml)

 Basis of derivation



No illness seen in most epidemiological studies



Upper threshold is above the threshold of illness transmission reported in most studies



Represents a substantial elevation in the probability of adverse health outcomes



Above this level there may be a significant risk of high levels of illness transmission

Calculating the microbial assessment category

The 95th percentile values for each of the categories were determined by the World Health Organization using enterococci data from European swimming locations. These values will show different probabilities of illness if the distribution of enterococci data at swimming locations in NSW differs from the European distribution. Due to this issue, Beachwatch uses the Enterotester tool (developed by Dr Richard Lugg, Department of Health, Western Australia) to calculate a modified 95th percentile that represents Australian conditions.

The Enterotester tool is available for download from the WA Health website.

View the latest State of the Beaches report to see the microbial assessment category for your local beach.