A Wellington bank officer expects to be the first person to apply for flexible working hours when a controversial new law takes effect today.

The 43-year-old loans processing officer, who cannot speak to the media under the rules of his employer, the Bank of New Zealand, has had a pilot period working Mondays to Thursdays so he can share childcare with his wife, a nurse who works Fridays to Sundays.

But Andrew Campbell of the financial sector union Finsec said that when the pilot ended the bank told him to go back to a five-day week, despite the new flexible working hours law.

The new law, introduced by Green MP Sue Kedgley but taken up by the Labour Government, allows all employees with caring responsibilities who have worked for the same employer for at least six months to request flexible working arrangements from today.

Employers must respond within three months, either granting the request or specifying reasons they cannot agree - such as their inability to reorganise work by other staff or to recruit extra staff to cope.

The law was opposed by business groups and the National Party, but David Lowe of the Employers and Manufacturers Association (Northern) said yesterday that there had been little interest from employers in attending training sessions on the change.

"Workplace flexibility has long been a key issue for us, so what's changed?"

Mr Campbell said the BNZ's insistence that its employee had to work his 37.5 hours a week over five days instead of four would leave the man and his wife "significantly out of pocket" because of the need to pay for childcare on Fridays for their children aged 5 and 2.

"What he's been doing on Fridays is dropping his son at school, taking the other child to Playcentre, taking them to swimming lessons, putting them to bed, spending time with his wife," Mr Campbell said.

"For a company that made $300 million profit for the first six months of this year to say that they can't introduce some degree of flexibility, we think is pretty outrageous."

But the BNZ's general manager of people and corporate relations, Bridget O'Shannessey, said the bank had offered the man a compromise in the form of other options for more flexible work.

She said the bank was actually "passionate" about flexible working and that was why it introduced the pilot scheme last October. The pilot would be extended to all 6000 BNZ employees from tomorrow.

"Flexible working is about making sure we are able to provide our customers with a service everywhere and at every time of the day, and that we are able to provide our staff with a really good work life," she said.