The new Foreign Investment Fund tax regime is a complicated nightmare likely to cost DIY investors time and money, says Wellington taxation software developer Tony Ryburn.

And an online Internal Revenue calculator provided to help owners of offshore shares produce returns has "serious shortcomings," Mr Ryburn said.

"With the July 7 tax deadline looming, we believe many DIY shareholders will be forced to seek expensive special assistance if they own shares in Australia," he said.

"While the basics of the FIF regime seem relatively straightforward, there is considerable underlying complexity and we soon realised that the range of information needed to complete tax returns is mind-boggling."

Mr Ryburn last month launched a new online portfolio management tool, Sharesight, which automatically calculates true returns and produces all relevant data for taxation requirements.

He has since developed the software to simplify FIF returns.

"Once a portfolio has been entered into Sharesight, the user is provided with a sophisticated but easy to use report which calculates taxable FIF income," he said.

"This takes only a few minutes - compared to the IRD calculator which can take hours - because most of the required information is entered automatically from a database of historic share prices, exchange rates and dividend payouts.

"You have to enter data into the IRD calculator one share at a time, which is very time-consuming, and it is not saved so if a correction is needed all information from the entire portfolio has to be re-entered."

Mr Ryburn said the new rules generally exempted investments of less than $50,000 but working out if exemptions applied could be a complex task if multiple trades of one particular shareholding had been made.

"Share splits and bonus issues add further complication," he said.

"An exemption applies to some Australian shares but the rules surrounding which shares are exempt are unclear."