The Labour Party is calling for assurances that privacy breaches will not happen again after Inland Revenue mistakenly sent one of its customers personal information about 27 people.
The IRD says documents were put into the wrong envelope in its child support area and sent to a customer.
The department has apologised to the people affected.
Labour's revenue spokesperson David Clark says the error shows the IRD is over-stretched and its computer system is out of date.
"An organisation that and update its computer systems has a serious problem. We know that every department is reliant on computers these days.
"This one's built in 1992, it's no longer current ... we need to see a plan from the Government and a timeline for this update."
The matter has been referred to the Privacy Commissioner.