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.
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"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.
When the service is relaunched on Tuesday, users will be able to identify a number of changes.
The most noticeable of these will come in the shape of the new "My Business" section in myIR, which will allow business owners to file, pay and amend GST as well as fringe benefit tax and gaming machine duty.
A new provisional tax option, called AIM or the Accounting Income Method, has also been introduced this month, with the effect of ensuring that small businesses will only have to pay provisional tax when they are making a profit.
"We are building up a set of products that are all designed to make tax simpler," said James.
"AIM will be a cash flow game changer while the new "My Business" section brings more tax types online and all in the one place."
Almost 14,000 people have attended either online presentations or seminars around the country to preview the improvements.