The rally, organised by the New Zealand Council of Trade Unions (CTU), marked the start of a campaign for fairness at work.
CTU secretary Peter Conway addressed the crowd of about 500 in the Octagon, after it marched up George St, and spoke about the right of all workers to decent pay and conditions and to be treated with respect.
He argued against the Government's move to extend the 90-day probation law to all businesses, to restrict union access to workplaces, and force workers to produce medical certificates for every day they were unwell.
The changes were "a disgrace" and a sign the Government had "turned against the workers".
The Bill to enact the changes passed its first reading in Parliament on Thursday 64-57 after strident protests from the Labour Party.
The probation period now covers businesses with 19 or fewer employees.
Unions say when it covers all businesses it would affect 400,000 workers - the number who start work or change jobs each year.
During the probation period, employees can be sacked without the right to claim unfair dismissal.
The Government says it has already benefited hundreds of unemployed people who would not have been hired without it.
If the Bill becomes law, union access will be conditional on the consent of the employer. Rally participants were treated to a performance from American singer-songwriter David Rovics, and heard from workers concerned about the changes.
Jo Hunter, a teacher, said she was "sick and tired" of the "insidious creep in the erosion of workers' rights", and Dave Kearns called on the group to "change the Government".
"John Key knows this is not about fairness. It's about giving all the power to one side of the table, the side that his friends sit on," Mr Kearns said.
Mr Conway urged people to make submissions on the Bill, sign petitions and to continue talking about the changes to raise awareness.
Other rallies were held in Auckland, Wellington and Christchurch on Saturday.
A national day of action has been planned for October 20.
