A global software update failure by cybersecurity firm CrowdStrike that was deployed on Friday night (NZ time) took out computer systems belonging to airlines, healthcare providers, retailers, news outlets, broadcasters, financial outlets, infrastructure and transport networks and more.
CrowdStrike says the bug is fixed - but it could be some time before all systems are back up and running.
Mitchell said in New Zealand essential services were still working.
"I've been informed that it's minor impacts that don't relate to critical systems."
A patch was available straight away, he said.
RNZ has been told by a doctor that an integral part of the medical system was down last night. While it was not a life and death situation, they said it was causing significant delays and compromising patient safety.
Te Whatu Ora/Health New Zealand on Saturday morning declined RNZ's request for an interview.
St John and Wellington Free Ambulance's communications centres were both affected by the outage.
Mitchell said St John had to quickly move to a manual system to ensure there was no disruption to services. Action was taken immediately, with staff using VHF radio and writing notes on paper, he said.
"They're back to business as usual this morning."
Mitchell said it was not good enough that the update was faulty.
"There's a lot of money paid for this system and it's a very expensive system and so that's not good enough and obviously... as a government we'll be wanting some answers around that."
But there was an outstanding response from all of the agencies as soon as they became aware of the CrowdStrike fault, he said.
Te Whatu Ora/Health NZ would need to provide any detailed information about the extent of the disruption in the health sector, he said.
"We were just interested in making sure that we minimised the impacts, that critical services were still being provided - and they were."
Mitchell said there was an immediate government response.
"As soon as government became aware of the problem [the National Emergency Management Agency] was stood up as the lead agency, there was work across [Fire and Emergency], police, ambulance services, [Civil Aviation Authority], our utility providers to ensure and make sure that as a country we had the best possible response in place, that we were looking at the horizon to try and anticipate what other issues may emerge for us as a country and how we deal with those."
New Zealand had not been impacted as badly as other countries for something that was a massive global outage, he said.
Overseas airlines had been affected so those travelling should check for any possible disruptions or delays as a result of the outage in other countries, he said.
"But as of this morning, we're in a very good space, most systems have been restored and are back up and running and the ones that aren't should be back to business as usual by the end of the day."
The recent Rū Whenua exercise which dealt with the scenario of a major alpine fault in the Fiordland area, had been "very effective", and the government would continue to run these "so we're best prepared as a country to deal with any of these types of incidents", he said.
The outage was a reminder that any piece of infrastructure does carry vulnerabilities, he said.
