Investors may have been paying too much tax for over a year, thanks to a faulty Inland Revenue online calculator.

Inland Revenue’s now admitted it was told in September about it producing wrong results - but didn't end up fixing it.

Deloitte tax partner Robyn Walker told Heather du Plessis-Allan it's disappointing it got to this point, but it's hard to know how wide of an impact it will have as it is a niche market. 

"This is for investors who are putting money into foreign shares. They are investing over $50,000, and they are subject to a regime called the Fair Dividend Regime - and within that, there's a specific rule and calculation that says if you buy and sell shares within a 12 month period, you have to do what's called a 'quick sale adjustment', and it's this element of the formula where we think all the problems are arising from."

She says the taxpayers who were buying and selling in that period are most likely to be affected. 

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