Dairy company Parmalat has locked about 60 workers out of a processing plant in northern Victoria indefinitely amid a heated dispute over pay and conditions.

Negotiations for a new enterprise agreement at the Echuca plant have been underway since August 2016, and in November, Parmalat applied to have the existing agreement torn up.

The Australian Manufacturing Workers Union says that would slash workers' wages by close to half and cut redundancy and other entitlements.

AMWU and Electrical Trades Union workers voted to go on a protected four-hour strike starting 3am Wednesday, but arrived at the factory to find themselves locked out.