A Christchurch-based environmental consultancy has found swimmability in Horizons Region waterways improved 5 to 8 per cent between 2006 and 2016.

The independent study was by LWP, and commissioned by Horizons Regional Council and the Ministry for the Environment. It was part of the information needed to determine whether progress is being made on government's swimmability targets.

It focused on just two factors that influence swimmability - the sediment that affects water clarity and the presence of the faecal coli form bacteria E.coli.

Other water quality factors, such as nutrient loading and the health of freshwater life, will be looked at in time.

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The report found a whole array of changes had contributed to the improvement. Among them were upgrading sewage and factory discharges, fencing and planting waterways and planting the most erodible land.

There was weak but significant evidence that the council's Sustainable Land Use Initiative (SLUI) was making a difference. It uses voluntary farm plans that now cover 500,000ha and have resulted in 14 million trees being planted.

If present progress continues the SLUI will keep 27 per cent of current sediment out of waterways by 2040, councillor David Cotton said.

The evidence that upgrades to sewage and factory discharges are making a difference is stronger. And as old consents are renewed and stronger conditions are imposed, that improvement should continue.

Scientists expect any changes to take years to improve water quality. They have been pleasantly surprised, Mr Cotton said.

Fellow councillor Rachel Keedwell said it was good to see improvements and more factors were moving in the right direction than in the wrong one.

But she said the study was looking at changes over time, rather than the actual state of the water.

"There are still spots that are swimmable less than half of the time."

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On Monday the LAWA website advised caution to people swimming at Whanganui's coastal lakes, the Manganui O Te Ao at the Ruatiti Domain, the Mangawhero at two places and the Whanganui at the Town Bridge.

People were being advised not to swim at all in coastal streams, especially the Mowhanau at Kai Iwi Beach, where a long-term notice is in place due to "moderate to high risk of illness".