And if long-term proposals go ahead to extract water to supply Geelong, Warrnambool's water could be at risk.

Authority Barwon Water was yesterday unavailable to comment but has maintained its exploration of the Otway Ranges was merely an "information gathering exercise".

The concerns, from Gellibrand area landholder Malcolm Gardiner, come after water tests were conducted at Deakin University's Warrnambool campus.

Tests found water taken from one point along Boundary Creek, near Forrest, had a pH level of 2.7, which is 10,000 times the neutral level of 7, Mr Gardiner said. Rainwater has a pH level of about 6.

A fierce opponent of Barwon Water's groundwater extraction in the Otway Ranges, Mr Gardiner said he developed severe diarrhoea after drinking a handful of water from Boundary Creek.

"When you drop the water levels, the peat that's normally saturated with water . . . becomes acidic," he said.

The acid sulphate soils leached into the water, increasing the pH level, he said.

Mr Gardiner warned that Barwon Water's proposal to take water from the Newlingrook aquifer to assess its potential to supply Geelong residents, could threaten the south-west's water supply.

The Newlingrook aquifer is linked to the Gellibrand River, which supplies Warrnambool.