A customer who did not wish to be named said she had tried to contact IRD this morning with a provisional tax query.
She said she phoned both the 0800 number and a Wellington number and was told to enter her details and had to wait for awhile before she was told the computer systems were down.
She said the helpline was also overloaded whenever a tax date loomed. Provisional tax payments are due on May 7.
"As a business owner this is very frustrating - earlier in the year the IRD's telephone staff told me I had a provisional tax payment due on May 7.
"Even if the computer is down the system should still take people's names and numbers and call them back,'' she said.
A spokesperson for Revenue Minister Peter Dunne said the problem had been caused by some staff members not being able to log in this morning.
A spokesman for the department said that due to a technical problem, some staff were not about to receive or send email.
"However, the service has been fully restored. This incident is not related to the recent issue with our email gateway,'' the spokesman said.
Documents released yesterday showed the IRD computer system had been at risk for at least three years and seriously in need of the $1.5b upgrade.
Mr Dunne yesterday announced the upgrade - the largest overhaul of any government IT system.
He said it was "fully stretched'' and a 10-year project to upgrade the system was required.
The upgrade is scheduled to take 10 years, but could be longer.
Mr Dunne said IRD's functions had changed since its computer systems were launched in 1991.
