Alcoa's aluminium smelter in Victoria will be shuttered from Friday, the factory closing after more than 40 years of operation.
The final production lines will be cleared and the workforce will clock off for the final time.
About 500 workers have lost their jobs.
"It is a day of unspeakable sadness for most and brings with it fears for the future for a great many," AWU Victorian Secretary Ben Davis said in a statement.
"Workers have been dealing with the reality of the closure and what it means for their lives since Alcoa announced its decision in February this year."
The union also said a survey of the redundant workers found a "limited number" had secured new jobs and a further 20 per cent had "firm prospects".
Alcoa has announced it will close its Point Henry smelter, a rolling mill in Geelong and its Yennora rolling mill and recycling facility in NSW, by the end of 2014.
