Anti-fluoride extremists have threatened to kill a Victorian government minister and blow up a regional water authority.

As anger over fluoridating Geelong's water supply mounted today, Labor minister Lisa Neville has been targeted along with water and health officials.

A death threat was left with a bottle of water on the verandah of Ms Neville's house on Saturday night, a government spokeswoman said.

Anti-fluoride activists have also threatened to blow up Barwon Water's treatment plants as the authority begins on Monday adding fluoride to the water supply in Geelong, 70 kilometres southwest of Melbourne.

The Department of Human Services (DHS) received a threatening letter in the mail last week.