The Inland Revenue Department (IRD) website and customer services will be unavailable for four days this week as the organisation rolls out major system upgrades.
The myIR site will be down from Thursday afternoon at 3pm until Tuesday April 17 at 8am to allow for the changes to be incorporated.
IRD contact centres and front of house services will also be closed during this period, but customers will still be able to pay accounts using online banking.
ir-File, used by businesses to submit Employer Monthly Schedules, will be unavailable from midnight on Wednesday.
"Moving these tax types and the associated data over to our new computer system is no small task so we're taking the system offline to allow a smooth transition," said deputy commissioner of transformation Greg James.