Melbourne union rally crowd expected to be 15 times bigger than Sydney. Why?

And this week a number of conferences for union members are being held in the Victorian capital, including United Voice Victoria which is holding its delegates and leaders convention in the CBD on Tuesday.

But former head of the Workplace Research Centre and head of business analytics at the University of Sydney Business School, Professor John Buchanan, said the difference in the rallies is a reflection of historical traditions of activism in both states.

"Politically, Victoria has always been more polarised so going back a long way, the Australian labour movement was important for setting the standard for an eight hour day and that came out of disputes from the stone masons in Victoria in the 1850s," he said.

"NSW has always not been far behind but Victoria often did set the standard."

Professor Buchanan said a history of more conservative state governments in Victoria and Labor governments in NSW had also shaped the way unions in each state use different means to achieve their goals.

"These are long traditions which go back in some cases centuries. But in more recent decades, the current Victorian Trades Hall is under an activist leadership. The NSW Labour Council and Unions NSW are also very good but they have often been more successful in using parliamentary means to achieve their ends," he said.

"There is a longer tradition of Labor governments in NSW, and they built their customs and practices around that. In Victoria there is more of a tradition of conservative governments so unions have had to have recourse to direct action to enforce their claims."

Victorian Trades Hall secretary Luke Hilakari said the different crowds reflected the history of the union movements in both states.

"We have a larger tradition than NSW about hitting the streets," he said.

"It goes back to the days of Eureka, radical action. When it came to action to make changes to working people, Victoria has always been stronger in that space. Victoria was the first place on earth to win an eight-hour day, it was Victoria where the first parliament ended up meeting.

"When you think of large protests on the Iraq War, Vietnam, there is a long tradition of people willing to put boots on the street to stand with each other to come together collectively when we have important moments to make change."

He said the fact the federal ACTU and other union headquarters were located in Melbourne also played a role.

"There's almost a centralisation of progressive organisations and politics in this state. This state is probably the most progressive in our nation. It's gets reflected in the actions the populace is willing to take."

An ACTU spokesperson said while Melbourne was expected to host the largest crowd, each rally played its part in sending an overall message to the federal government.

“Change the Rules protest rallies are happening across the country, and every single person attending one is doing something important and courageous for our country’s future," a spokesperson said.

“While Melbourne may be the largest, it is no more important than those happening today in Darwin, Sydney, Wollongong, Mackay, Rockhampton, Townsville, Cairns or Gladstone. Or those later in the week in Hobart, Launceston, Devonport, Burnie, Shepparton, Adelaide, Wodonga, Bendigo, or Newcastle. Or those next month in Brisbane and Canberra,” he said.

The Melbourne rally began at Trades Hall, on the corner of Lygon and Victoria streets, about 10.30am and protesters were marching through the city from there.

The rally is expected to close roads and disrupt public transport, including trams and buses.

VicRoads has also advised motorists to plan for delays and if possible and avoid the CBD from 10am until after the rally finishes.

The Sydney rally will start at midday on Tuesday at Belmore Park near Central Station. The rally will head up Pitt Street, along Bathurst Street, down Castlereagh Street before it returns to Belmore Park.

Road closures will be in place from 12.15pm to 1.30pm on sections of Castlereagh St, Campbell St, Goulburn St, Liverpool St and Pitt St.

Unions NSW has not responded to requests for comment.