Union members yesterday turned in hope to Auckland mayor-elect Len Brown as they vowed to defeat the Government's new industrial laws at next year's election.

About 6000 workers, bused into the Manukau's TelstraClear Pacific Events Centre from all over Auckland, roared twice - once when a message of support from Mr Brown was read out, and again when National Distribution Union secretary Robert Reid said Mr Brown's victory showed National Prime Minister John Key was vulnerable.

He said Mr Key might have the numbers in Parliament to pass his controversial industrial law changes next month, but unions would not give up. "We will defeat this bill, nothing is surer!"

"If not today, we will defeat it tomorrow; if not tomorrow, we will defeat it on the streets, we will defeat it in the workplaces, we will defeat it next year when we throw this anti-union Government out.

"We defeated John Banks and got Len Brown in. In Wellington and Dunedin they also got rid of Tory mayors and got good pro-worker people in. We can win, we will win, we will throw this bill and this Government into the dustbin of history!"

A spokesman said Mr Brown was too busy preparing for his November 1 "inauguration" to attend the rally, but Wellington's new Green mayor Celia Wade-Brown got a rousing reception from about 3000 people at a similar rally outside Parliament.

Altogether the Council of Trade Unions claimed a turnout of 22,000 at 28 rallies yesterday against the proposed industrial laws including 1500 in Hamilton, 800 in Hastings, 600 in Rotorua and 250 each in Whangarei and Tauranga. But the Herald independently estimated the turnouts at 6000 in Manukau (the union claimed 7000) and 3000 in Wellington (compared with a claimed 4000).

Pacific Events Centre operations manager Talei Mattner said the venue had an indoor standing capacity of 4000 and a further 200 people were in an upper lounge. The Herald counted 1300 more people listening outside, and bus companies confirmed at least three buses arrived just as the event was ending.

Most workers interviewed said they were there mainly to oppose the extension of the 90-day trial period in which employers will be able to dismiss new workers without fear of unfair dismissal claims. "We think the National Government's agenda is to casualise all work in New Zealand and that is not a fair deal," said Sa Leutele, 52, from Bluebird's Wiri food plant.

Watersider Graham McKean, 50, said port companies were already trying to casualise work and contract more work out.

"We're fighting it every day."

The Manukau turnout attracted fewer people than a pre-event forecast by Service Workers Union official Jill Ovens of 10,000, but was higher than at any other union event since the protests against the Employment Contracts Act in 1991, when 10,000 people crowded into Aotea Square and more than 100,000 protested nationally in one day.

The Engineering, Printing and Manufacturing Union (EPMU) claimed 6000 at Manukau and 2500 at North Harbour Stadium in two rallies just before the 2008 election, but its estimate for the Manukau event appears to have been an overestimate because people did not spill outside the events centre as they did yesterday.

Political commentator Chris Trotter, who attended yesterday, said the sea of largely brown faces and multiple languages in the audience showed that there were now "two New Zealands".

"I'm not sure that Pakeha New Zealand has a very good grip on the other New Zealand, which is overwhelmingly Maori and Pasifika and immigrant," he said. "The two references to Len Brown drew very enthusiastic responses and I thought 'these people have got a taste of it now and they're going to do this again'. If they do, they will redraw the electoral map."

But Mr Key dismissed the rallies as relatively small. "I'm not terribly concerned about it at all."

He said the Government was changing the law to create more job opportunities and make sure New Zealanders could participate in the labour markets.