"The Delta variant spreads quickly and we have to be very, very cautious because of that," Grant Robertson told TVNZ's Q+A.
He said all of Australia was potentially at risk from the highly transmissible variant and that was why the entire travel bubble was paused for at least eight weeks, and not taken in a state-by-state approach.
"It's so transmissible. We've seen with Victoria ... once it gets in there it's difficult to contain.
"We need to be confident that it's contained and under control. Eight weeks gives us a window so that we can have another look at it."
Robertson said containing the virus did not mean having zero cases, but being able to "reliably predict where the cases will be", such as inside one household.
He said the Delta variant had "fundamentally changed the world's approach to Covid-19".
The Government announced it was pausing the transtasman bubble on Friday after a spike in cases across Australia and the spread of the Delta variant increased.
New Zealanders across the ditch were told they had one week to arrive home.
Mercy flights were snapped up almost immediately and sold out.
Air New Zealand said it would continue to add flights to its schedule as demand became clearer.
In New South Wales (NSW), New Zealanders can only fly from Sydney and will have to be in MIQ for 14 days.
NSW reported 163 cases yesterday, 45 of whom had already been in the community while infectious.
In Victoria, New Zealand travellers must have a negative pre-departure test and must isolated until a negative day 3 test.
Of 12 new cases in the state overnight, all were from the Delta variant and the NSW "incursion", Victoria Health Minister Martin Foley said.
Robertson was still confident the transtasman travel bubble could continue, despite not all New Zealanders being vaccinated.
"We do believe it is possible to mange it in a situation where we are still vaccinating in New Zealand," he told Q+A.
"The advice we continue to get from officials is that is still manageable."
But University of Auckland medical professor Des Gorman has questioned the decision that travellers arriving back from states other than New South Wales will not need to enter managed isolation.
"This is an example of management by public relations rather than management by scientific risk," he told Newstalk ZB.
He said the Delta variant was hard to control.
"It gets ahead of contact tracing. Every time you think you got some idea of the number of people involved you find out, in fact, it's much greater than what you think."
Vaccinations
Robertson admitted the day-to-day rollout of the vaccination programme "could be better" but remained tight-lipped on exactly what issues could have run smoother.
Just under 300,000 people in group three had been vaccinated, according to data on Tuesday - about 20 per cent. Vaccinations for the first age band in group 4 are scheduled to begin on Wednesday.
Robertson said the "programme is running well" and was still on track to have all New Zealanders vaccinated by the end of the year.
"The feedback from public is that it is a good process," Robertson said.
"The percentages of group three will ramp up significantly with the booking system in place."
People aged 60 and over would be able to book their vaccinations from July 28, the Ministry of Health website said.
Advice for Kiwis in Australia
Travellers from Queensland, South Australia, Tasmania, Western Australia, ACT and Norfolk Island can come home on a green flight subject to the below public health criteria:
• A negative pre-departure test, taken within 72-hours of their intended travel to New Zealand.
• Have not been in a location of interest in the past 14 days.
• Are not symptomatic at the time of travel.
• Are not a contact of a Covid-19 case.
Eligible people from Victoria or travellers from other states/territories who have been in Victoria can return provided they also:
• Adhere to lockdown measures in Victoria.
• Self-isolate upon return to New Zealand and get a test at day 3.
• Travel to the airport wearing facemasks and by safe travel, ie, not public transport.
Eligible people from New South Wales will continue to return on existing managed return flights.
Returnees on these flights will be required to enter a managed isolation facility for at least 14 days on arrival in New Zealand.
Travel on all such flights will be limited to:
• New Zealand citizens and holders of residence class visas.
• Holders of temporary visas and Australian citizens, who last departed New Zealand after April 5, 2021.
• Holders of current permanent residence visas (including a resident return visa) issued by the Government of Australia who last departed New Zealand after April 5, 2021; and relevant family members of people listed in the above categories. (Relevant family member means: a spouse, civil union partner, or de facto partner, a dependent child; or a parent of a dependent child. Parent, in relation to a dependent child, means a person on whom the child is dependent).
