Roughly 182,000 litres of farm dairy effluent discharged into a tributary of the Mangatutu Stream, which flows into the Tutaekuri River, is expected to be cleaned through a "natural process".
The Tutaekuri River flows east from the Kaweka Range, reaching the Pacific Ocean just to the south of Napier, where the Ngaruroro and Clive Rivers join it.
On Tuesday the Land Air Water Aotearoa (Lawa) website noted the site monitored near the Hawke's Bay Expressway bridge, a popular swimming spot, failed a water quality test "due to a pollution incident".
The Hawke's Bay Regional Council will sample the water quality again on Monday, but until the council gets a "green" result back from routine water sampling, the Lawa status will stay red.
This means the site is unsuitable for swimming.