The rollout of a new Work and Income IT system has been beset by problems, resulting in delays for clients.
Social Development Minister Anne Tolley says the new client management system went live at 7am on Monday.
Just a few hours later, her office was advised the system was experiencing problems.
Ms Tolley says the system itself is performing well, but there are issues with capacity -- meaning the number of users who can be on the system at one time.
"We took some steps knowing that we would have a slow down this week, but it's been greater than was expected," she told reporters.
"There have been some delays and we know that that's frustrating for some people."
The system is now running at 90 percent capacity and it's expected to be at 100 percent by the end of the week, Ms Tolley said.
Everyone's payments have gone through, but people who had been expecting adjustments -- for example because of changes to work hours -- may not have had those processed.
"We'll get those changes made for them next week," Ms Tolley said.
The new client management system has been in development since 2014.
Ms Tolley said the system's capacity was tested before the rollout.
"It just shows when you go live, things don't always work out quite as you planned."
Labour's social development spokeswoman Carmel Sepuloni says the system problems have meant some people have not been able to access emergency food grants and housing.
"Anne Tolley must take responsibility for these failures as it is affecting a wide range of people including those applying for new benefits," she said.
Ms Sepuloni said some WINZ staff have reported being unable to access information or process grants and new benefits.