The Inland Revenue Department says it has fixed a system error which caused its website to crash for three hours on Monday, affecting thousands of people trying to file tax returns.
Monday is the deadline for goods and services tax (GST) and provisional tax returns of contractors, farms and businesses.
The IRD has apologised for the glitch and assured those trying to file their returns they will not be disadvantaged.
Acting commissioner Martin Smith says any customers who could not file on Monday have been asked to file as soon as they can.
People trying to phone IRD about the problem reported a wait of up to 90 minutes to get assistance.