'Mondayising' Anzac and Waitangi days will be debated in Parliament today with the introduction of Labour Party member David Clark's bill.
The proposed bill would make Monday a holiday if Anzac or Waitangi Day occurs on a weekend, as happened last year.
The next time either of the two holidays falls on a weekend will be in 2015 and both will fall at the weekend in 2021.
Prime Minister John Key this morning said the Department of Labour had estimated 1.4 million workers would be affected and observing either day on the following Monday would result in a $200 million burden on the economy, or $400 million if both fell on a weekend.
But he had asked for more advice as those figures seemed "superficial", according to Fairfax.
"Last year was the year we had both of them falling on the weekend and for some people that was pretty annoying," he told TV3's Firstline.
The CTU is supporting the legislation and says the effect on the economy would not be as extreme, as people tended to spend more on a long weekend, and most employers budgeted for the cost of Anzac Day and Waitangi Day falling on a working day in a normal year.
United Future, National's support partner, has also indicated it will back the bill.
Key acknowledged the line of thought that employers bore the cost of these holidays most years and that there were many small businesses struggling as it is.
"If you are a salary worker, it really doesn't make any difference because you get paid a certain amount anyway, and if you are on wages you get time-and-a-half and a day in lieu," he told Fairfax.
"Some people will argue it is built into the system, employers pay it most of the time ... so rather than costing the economy, it's a little bonus which goes the other way."