"Inland Revenue's systems are back online early after completing the latest round of Business Transformation work ahead of schedule," IRD said in a statement late this afternoon.
"The system was scheduled to be shutdown until 8am tomorrow but because the upgrade went really well, IR has been able to reinstate its myIR service and re-open applications for the Resurgence Support payment through myIR from now."
Earlier, businesses in need of urgent financial support were unable to apply for the Government's Covid-19 Resurgence Support Payment as IRD's website is down "for maintenance".
Yesterday, Finance Minister Grant Robertson's advised that businesses in need of support were to apply for the payment via IRD's MyIR login portal.
However, Inland Revenue's website appears to be down this weekend.
A message on its website reads: "The service is offline for routine maintenance. Try again later", causing frustration for business owners.
The system upgrade has been in effect since Friday evening and not due to be finished and the website back online until Monday.
Phone lines are also said to be currently unavailable.
National Party shadow treasurer Andrew Bayly is calling for Inland Revenue to scrap the maintenance plans and relaunch its website to help businesses, particularly those in Auckland, who face the financial burden of an alert level 3 lockdown - the second in a span of a month.
"It is essential the IRD opens to help people to calculate and lodge claims for their Resurgence Support Payment," Bayly said.
"The latest lockdown will be a devastating blow for businesses not only in Auckland but across the country."
IRD needed to sort out its systems immediately to ensure businesses could access immediate help, Bayly said.
"During times of financial stress it is vital that people are able to call government officials at IRD to discuss their options.
"IRD needs to sort out their systems immediately so New Zealanders can access the help they need."
A spokesman for IRD told the Herald the system would be back up and running in less than 24 hours from now.
"We did think seriously about timing and the possibility of the systems being shut down at a higher alert level, but in the end decided that because it was such a short shutdown people would not be materially disadvantaged," the spokesman said.
"IR understands it may be frustrating for some people but these are essential upgrades to our systems and services so that we can continue building a fairer tax system."
Finance Minister Grant Robertson has confirmed the latest lockdown triggers the wage subsidy scheme and that applies nationwide. This will be paid in a lump sum.
He said businesses could receive both the wage subsidy scheme and the new alert level 2 subsidy scheme.
Government had budgeted $400-$500 million for the coming seven days, Robertson said.
Based on previous lockdowns, Auckland at level 3 and the rest of NZ at level 2 costs the economy about $440 million a week, according to ASB.