Thousands of litres of raw sewage poured into the Beaulieu River flooding the entrance to the New Forest Golf Club.
Murky brown water was seen flowing through a brook after a Southern Water raw sewage main burst.
The beleagured utiltiies company - prosecuted repeatedly for failings - has apologised.
Officers at the Environment Agency are investigating the August incident and have taken samples from the river.
An independent expert told the BBC - which first reported the spill - that it was possible 1.2 million litres had flowed into the river over the course of two days.
Golf club bosses said the stench was 'unpleasant'.
Naturalist Chris Packam told the BBC: "It takes a lot to shock me.
"That this is happening in 2023 in one of the most nature protected parts of the UK - a National Park an SSSI - a site of special scientific interest - and on top of that a Ramsar site - all of these very important pieces of legislation to protect this area and its wildlife.
"That this is happening is an absolute disgrace."
The Envinronment Agency is investigating. Samples are being taken from the water.
A stement said: "Our officers were on site at Lyndhurst for almost a week from 13th August to mitigate and limit any environmental harm, and gather evidence for our investigation.
“Members of the public who suspect pollution or spot fish in distress should report it to our 24-hour incident hotline immediately on 0800 80 70 60.”
Southern Water has apologised.
A Southern Water worker told the BBC: "We absolutely made errors in not getting hold of some our key stakeholders quick enough - that's something we've done a deep investigation into and (with many of those key stakeholders now we've put processes in that should anything like this happen again - which we're working hard to make sure it never does - that they're informed at a much earlier date in the incident."
He added: "In instances like this we don't measure the volumes that are going in as it's an uncontrolled nature, what we do more importantly is measure the impact on the environment."
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