The amount of money expected to be raised by an incoming sugar tax has been overestimated by almost €100 million.

It's expected the incoming levy on sugary drinks will yield around €40 million per year, rather than the €134 million predicted in 2014, the Sunday Business Post reports.

The Department of Finance's tax strategy group admit they relied on "imperfect" and "speculative approxomations" in calculating that initial estimate.

It's also believed that soft drink manufacturers have altered their recipes to reduce sugar content and avoid the tax.