Director-General of Health Dr Ashley Bloomfield has admitted "rapid work" was required for the health system to switch from elimination of COVID-19 as the outbreak spread in Auckland.
His comments came after Health Minister Andrew Little told Newshub's Michael Morrah the home isolation system "wasn't ready" to cope with more than 120 daily cases of COVID-19 in the community.
Little's remarks led to criticism from the Opposition that the Government was too focused on eliminating COVID-19, rather than planning for a scenario in which it could not be contained.
Prime Minister Jacinda Ardern announced on August 17 that New Zealand would enter a "short and sharp" lockdown to contain the spread of the Delta variant detected in an arrival from Australia in Auckland.
But as Auckland entered its fifth week in alert level 4 and cases were still emerging, Ardern announced the decision to move the city into alert level 3 the following week. From there, modelling shows the cases continued to climb.
As Auckland experiences its 14th week of lockdown, it's now evident that the virus is spreading through the country, with cases detected in Northland, Waikato, Rotorua, Taupo and Wairarapa - despite Auckland border checks.
There are now thousands of COVID-19 cases isolating at home, after managed isolation and quarantine (MIQ) facilities quickly reached capacity.
Dr Bloomfield told Parliament's Health Select Committee on Wednesday the health system was forced to rapidly adapt to the change from eradication.
"We were ready for an outbreak response which is what we had done, with a significant surge in people in quarantine and then going back down to elimination approach again, and we've had to switch that," he said.
"The system was ready to a very large degree but then there has been some very rapid work that has needed to happen to help support that switch."
Dr Bloomfield said there was "very active planning underway to be able to sustain" the eradication response.
"Our testing, contact tracing, quarantine and isolation system has proven to be very effective over the last 20 months at responding rapidly to outbreaks and getting us back down to zero - and that's what we were primarily geared up to do.
"The key thing here is we're switching the system across all those dimensions - testing, contact tracing, support in the community, hospital capacity and how they're geared up - to what I've been describing as a sustained and sustainable response, where we need to be able to do all of those things and the health system can continue to deliver all the other things people need.
"It's different doing that when we're not in an alert level 3 or 4, where things like all planned care is stopped. We want to be able to maintain the planned care and still deal with the outbreak.
"So yeah, it is a switch for the system, we did quite a lot of preparation for that. But now we're in the process of doing the rest of that preparation as we can see the outbreak will start to move across the rest of the country."
Dr Bloomfield said modelling shows there will be 200 about cases a day by the end of November, and thanks to high vaccination rates, "our hope is that it starts to level off at that level".