Up to half a million taxpayers have had tax bills written off because of additional paydays - and many may not have been aware of it.

Some teachers have been handed tax bills for the most recent tax year because they received an extra fortnightly pay.

Inland Revenue writes off up to $420 of tax for people paid fortnightly or $250 for people paid weekly, if it is owing only because of a quirk of the calendar.

But people who have other income that is not correctly taxed, or who receive Working for Families, use a tailored or incorrect tax code or move employers do not receive the write-off.

An Official Information Act request shows that in the year to March 189,200 people had a combined $30.9 million of tax written off for this reason.

The year before 58,100 people were responsible for $6.4m written off. In the year to March 2020, 510,500 people had $37.4m written off.

Inland Revenue said that was due to NZ Super recipients having an extra payday that year.

Deloitte tax partner Robyn Walker said the problem was that all PAYE tax was taken on the basis that there were 52 paydays, or 26 for people paid fortnightly.

But, depending on how dates fall, people can have 53 or 27 paydays.

She said in 2020, people who were paid on both Mondays and Tuesdays probably received an extra pay because it was also a leap year.

Otherwise, there were higher numbers in years where there were 53 Tuesdays, Wednesdays and Thursdays because they were more common pay days.

"A lot of the public sector is paid on a Tuesday."

She said there was probably no better way to manage the issue.

"However, much like predicting eclipses of the moon, watch this space in March 2032. In 2031-2032 there is a leap year again which will result in two Tuesdays and two Wednesdays... so that might be a year with much higher numbers of write-offs required."