The First Union says many of the rights celebrated with the public holiday on Labour Day are in danger of being lost due to planned law changes.
The union says Labour Day marks rights, like the eight hour day, that were won with collective action.
However, general secretary Robert Reid said changes to the law expected before Parliament this year, will erode the ability of employees to band together.
Mr Reid said the changes would include allowing employers to opt out of multi-employer bargaining and removing their obligation to conclude collective bargaining.
He also said it was a slap in the face that legislation extending the payment of a youth wage passed its first reading in Parliament in the week leading up to Labour Day.