The National Distribution Union says new figures show workers who are unionised receive greater pay rises.
Research from Victoria University shows more than 100,000 workers, who were part of collective bargaining, received an average wage increase of 4.2% in the year to June.
That compares to an average wage increase of 1.6% for all workers in the same period.
NDU General Secretary Robert Reid says the unions are helping to narrow the wage gap with Australia, which is a priority for the Government.
But the Employers and Manufacturers Association says that claim is too good to be true.
Spokesperson David Lowe told Morning Report there are problems with the information gathered, including that some of the agreements cover several years.
Mr Reid says it makes no sense for the Government to place limitations on unions, like restricting their access to work places.
Union access to work places is being considered by the Government along with other labour law reforms.