ANTI-FLUORIDE extremists are claimed to be behind a threat to kill a Victorian cabinet minister and sabotage Geelong's water supply.

A threatening note scrawled on a water bottle was left on Geelong-based Community Services Minister Lisa Neville's verandah on Saturday night. The note read: "Thanks for the poison, bitch -- ready to kill you slowly."

Threats had previously been made to attack Geelong's water supply before moves from local water authority Barwon Water to begin adding fluoride to the water this week.

Ms Neville spoke out yesterday to condemn the threat against her, which she blamed on extreme elements of the anti-fluoride lobby.

"What we have seen is some extremists who have threatened the water supply of Geelong as well as made threats against Barwon Water staff," Ms Neville said.

"Unfortunately on the weekend they crossed another line and threatened myself personally by delivering a message on the front veranda of my property. That is a significant line to cross and I am particularly concerned that my son, like every other child in the community, is safe in their own home.

"Extremist behaviour like this is completely unacceptable and we would expect that Victoria Police will use the full force of the law against these people."

Ms Neville did not know if the substance in the bottle was water or not but said police had seized it as evidence.

Acting Senior Sergeant Tony Francis of Geelong police said the investigation into the incident was following a "strong avenue of inquiry".

"Hopefully an offender will be identified in coming days," Sergeant Francis said.

Officers are expected to perform a security audit of Ms Neville's home and establish extra patrols in the area to help ensure her safety and that of her 12-year-old son.

The decision to bring Geelong into line with most of the rest of the state in having a fluoridated water supply was made after an 18-month debate, which Ms Neville said had turned nasty only in recent weeks.

Barwon Association for Freedom from Fluoridation has denied responsibility for the threats. The group has condemned the action and says its members support only lawful protest.

BAFF president Keith Oakley said there was no chance any of the group's 200 members was involved and the threat might have been from its opponents. "I've often worried if someone is trying to do it on the pro-side to discredit us," Mr Oakley said.

Geelong Mayor John Mitchell condemned the threats as disgusting and un-Australian.

While personally opposed to fluoridation, he said protesters should accept the decision and move on. "I'm disgusted by it," Mr Mitchell said. "Everyone has the legal right to protest and vent their views in accordance with the law. I hope whoever has done this, the full force of the law comes down on them because this is un-Australian."