Inland Revenue is experiencing problems with its online service, myIR.
Customers are unable to log-on to the online self-service portal, or are finding the log-on process very slow.
"We are working as a matter of urgency on a way to restore the service," a spokeswoman said.
IRD first noticed the issues yesterday morning. The problems are related to a systems upgrade over the weekend.
The agency could not immediately provide an estimated time for fixing the glitch.
Inland Revenue is part-way through a multi-billion, multi-year upgrade of its systems.
"In the last half hour [to 11.40am] things have improved with it but we still don't have an eta for a fix," the spokeswoman said.