ACTU secretary Greg Combet told the rally Holden's intentions are directly linked to Australia's free trade agreement with Thailand.
"[Prime Minister] John Howard says 'I am the best friend the Australian worker have ever had'. Well, how good is he to you," Mr Combet said.
"Right now we're looking at another 120 job losses here, now that's the worst thing that you can ever confront at your work.
"You lob in there and you're told sorry, you know, gotta get the product from Thailand now, 120 people to go."