When Sports Direct caved into pressure earlier this week and announced it would offer its 18,250 shop staff currently on zero-hours contracts the option of a guaranteed 12-hour week, the move was widely welcomed.

But the move will hardly dent the vast, and fast-growing, army of workers across the country who now have to live on insecure employment terms with no holiday or sick pay – and who Theresa May has pledged to help. New data on Thursday showed the number of UK workers on zero-hours contracts has leapt 20% in a year to more than 900,000 – or 2.9% of the working population.

Employers’ groups insist they are welcomed by students and part-time workers and an essential tool for modern management.

But six months ago, on the other side of the world, a rather tougher line was adopted that May might just be studying.

The New Zealand government abolished zero-hours contracts – and the business world is having to find new ways of working.

“Since the legislation was brought in banning zero-hours contracts, corporates realise they can’t defend worker exploitation any more,” said Gerard Hehir, national secretary of the Unite union in New Zealand.

“The employees’ welfare has become a priority, not an afterthought. They are not just arguing with a puny little union any more – we have the law on our side now.”

It was on 11 March that the New Zealand government unanimously passed legislation banning zero-hours contracts across the nation of 4.5 million people.

“This is an incredible victory and I am still shocked by it to be honest,” said Unite’s0 national director, Mike Treen, when the law was passed. “The fact that the ban was unanimously supported in parliament is pretty unbelievable.”

Treen estimates hundreds of thousands of Kiwis were employed on zero-hours contracts, which demanded availability for seven days a week with no guaranteed hours. As in the UK, the contracts were especially insidious in the fast-food, cinema, hotel and casino industries. They included big names such as McDonald’s, Pizza Hut and the hamburger chain Wendy’s.

From 1 April it became illegal for any new workers employed to be offered zero-hours contracts and any current workers must be offered compliant employment terms by 1 April 2017.

Unite members have since been run busy negotiating new employment contracts for their members – and travelling internationally to share their success story with other unions.

The new legislation stipulates that workers must be offered a fixed, minimum number of hours each week, and will not be penalised for refusing any extra work they are offered at short notice.

“We have been pretty in demand, everyone wants the nitty-gritty details of how we pulled it off,” said Joe Carolan, a senior organiser for Unite.

“Internationally, activists think the speed of the victory was startling. But we’ve been campaigning since 2005. We’ve had long drawn-out scraps. Endless fights.”

This week the union has been celebrating the conclusion of a significant and lengthy negotiation with Restaurant Brands New Zealand (which includes the KFC, Starbucks and Pizza Hut franchises) agreeing to move its 4,000 zero-hours employees on to fixed-hour contracts. The company will now guarantee workers a minimum number of hours each week, and “reasonable compensation” for any time they are expected to be available.

Intense negotiations are also continuing with Burger King and Wendy’s, said Carolan. But one fast-food chain is continuing to dig its heels in: “McDonald’s are tough,” said Carolan.

Hehir said the zero-hours contracts demanding 24/7 availability from employees were evidence of poor management – and using modern technology most companies had been able to accurately pinpoint the busiest times, when extra workers may be needed, to a 10-hour window each week.

The introduction of minimum fixed hours had also taken away any tools some managers use to discriminate against workers on a personal basis – and reward or punish them with more or less hours arbitrarily.

“If you forget to smile at your boss on their birthday, that’s no longer something they can punish you for by withholding shifts,” said Hehir. “Workers have more stability, more confidence they will not be punished for personal slights – real or imagined.”

Shirley Wang, who is employed at Sky City Casino in Auckland on a fixed-hour contract and is an active Unite member, says the next hurdle is to move workers to banded employment contracts – in which the average hours they work are measured over a year, and the employee is then offered that many hours on a regular basis. Unite is also making moves towards unionising and defending migrant workers in New Zealand.

“The legislation was a massive win,” said Wang. “But that doesn’t mean there aren’t still huge challenges ahead of us. Before, no-one wanted to talk to us, we were dismissed. Now, businesses are finally realising that a lot of what we’re saying is actually common sense. That a more insecure, higher turnover of staff was actually costing them money. A loyal workforce is something every employer should strive for, because in the end it is always good for business. And that is our strongest argument.”

Carolan says what pushed the New Zealand ban on zero-hours contracts over the line was the swift, bipartisan support that became united by a common outrage: that Kiwi workers could not be guaranteed earning a living.

“What really ignited the campaign was when it became single-issue, and when we called it ‘zero hours’. The terms before that – underemployed, flexible hours – nothing resonated, But zero hours hit home. It’s not just about persuading and mobilising the workers – you’ve got to get everyone to see common sense. When we did, we were unstoppable.”