Chrissie Williams is a South New Brighton resident and a trustee of the Avon Heathcote Estuary Ihutai Trust.

With tussock-covered hills, braided rivers and sandy beaches, Canterbury has some outstanding natural features. Of all these riches, the Avon-Heathcote Estuary could be regarded as perhaps the most distinctive of all – and also the most vulnerable.

In the past 150 years more than 80 per cent of all New Zealand wetlands have been lost through draining and "development." The estuary of the Avon and Heathcote rivers is now the largest semi-enclosed shallow estuary in Canterbury and one of the most significant in the country. It is an internationally important habitat for migratory birds, and a major recreational playground.

Wetlands and estuaries are often referred to as "the kidneys of the planet". They filter and purify. The Avon Heathcote Estuary is a 700 hectare area adjacent to a city of around 400,000 people, and it receives a steady flow of contaminants from urban and rural catchments. Until the 1960s the Heathcote was used for disposal of wastewater from factories, abattoirs, a wool scour, fellmongery and various manufacturers – these types of discharges are much more carefully controlled.

The Estuary also receives a continuous discharge from the rivers and drains. Urban run off and stormwater contain all manner of compounds which can interfere with the estuary ecosystem and its food chain. Faecal and microbiological pollution from sewerage overflows, leaky pipes, septic tanks, and illegal sewerage connections contain pathogens which can cause campylobacter, salmonella, giardia, cryptosporidium and viruses. There are nearly 70 smaller stormwater outlets around the perimeter of the estuary, plus numerous small drainage pipes from surrounding properties, all delivering a mix of unwanted compounds.

The Estuary also receives twice-daily discharges from the Christchurch wastewater treatment plant which discharges load the estuary's water and sediments with nutrients which, when present in excessive amounts, can cause blooms of algae and disruption to the food chain.

In terms of human health, the estuary's water quality is officially graded "poor" or "very poor" for recreational purposes. Reports of infections among rowers or kayakers on the Avon are not uncommon – further evidence of the need to take a catchment-wide approach to protecting the estuary.

As for the health of the estuary ecosystem, some significant progress has been made. Lead levels have reduced markedly, largely due to the phasing out of lead in petrol, and high chromium levels have been reduced through regulations for trade waste. Life in and on the mudflats appears to have changed little. Cockles and other shellfish are abundant, but are still not suitable for consumption due to the faecal and heavy metal contamination.

The main area of concern for ecosystem health is excess nutrients – especially ammonia-nitrogen. Concentrations have increased two or three-fold over time at some sites, and up to eight-fold at Shag Rock.

Protecting the water quality of the Estuary is in everyone's interests. Every workplace and household can help by avoiding the use of kitchen waste disposal units (compost scraps instead) and cutting back on household cleaners containing ammonia, potassium and phosphorus. There are many natural alternatives. We can wash cars on grass to reduce soapy run off to the stormwater, and clean up after our pets to reduce faecal contamination when it rains.

Regular monitoring shows that the water quality in this delicate ecosystem has improved over the years. The commissioning of the ocean pipeline in 2008 for treated wastewater will bring further improvements. But discharges from the urban environment will continue to contaminate waterways and the estuary unless people are vigilant and take action.

The Estuary of the Avon and Heathcote Rivers – now the largest semi-enclosed shallow estuary in Canterbury and one of the most significant in the country.