The GST changes, part of a multi-million dollar transformation project, mean people will be able to pay and file their GST return at the same time rather than filing a return and paying through online banking at a later date.
But the shut-down has caused frustrations for people wanting to make tax payments ahead of a February 7 deadline.
While people can still pay their bill through their bank the shut-down of the myIR service and contact centre means people can not currently check how much they owe.
Customers with an outstanding debt due on the 7th could still pay through their normal method such as through their online banking, an IRD spokesperson said.
"There will be leeway for those who make payments."
He said people who paid on February 7 would not incur any penalties and the department would be putting on extra staff to deal with anyone's concerns.
The spokesman said it had communicated with around 350,000 taxpayers since January about the shut-down and there had also been information about it on its website.
The GST upgrade is part of a multi-year transformation project designed improve the tax department's systems.
Greg James, deputy commissioner transformation for the IRD, said the GST changes were the first tangible benefits for customers of Inland Revenue's transformation programme, and were being delivered ahead of schedule and within budget.
"The new capabilities will enable customers to register for GST, register as a preparer of tax returns, amend GST returns and accounts, file and pay GST at the same time, set up payment plans, and track their GST payments and refunds - all online."
According to the IRD more than half of New Zealand businesses file their GST through Inland Revenue's secure online service myIR, or direct from their accounting software.