Earlier in his brief leadership, Muller said the National Party believed Australian skiers should be able to come to New Zealand this winter.
But Muller said the "shambolic" response to testing people in quarantine of the past fortnight meant New Zealanders were now less willing to talk about how we might allow people to visit or travel internationally.
"It has reduced New Zealand's confidence in having a conversation, because even having a conversation right now creates the sense that 'oh, you mean we're going to open the border and get overwhelmed'.
"But you've got to start framing up what short to medium to long term country strategy is," Muller said.
"There has to be a greater standard in market, in terms of health checks, and confirmation of being free of Covid, than what's been the case, and you should be able to work with Australia and others to land what that looks like.
"Just because it is present in a country shouldn't mean that nobody from that country has the capacity to visit this country.
"We've got to be able to imagine a different engagement with the world than that."
Muller said it had to be possible to be able to have confirmation of people's health before they got on a plane and their health status when they got here, to the satisfaction of health and border officials in both countries.
"Then you should be able to have a conversation about what that means. Does that mean when they arrive two more weeks? Or is it a lesser time?"
Asked if he would be comfortable for visitors from any country, Muller said his point was that it was "simply untenable" to not be developing a plan on how to reintegrate with the rest of the world and the Government needed to communicate.
"We've got no visibility in terms of how they would manage the Australian situation, no visibility around the Pacific, despite the fact that most of those countries are without Covid.
"They can't manage the current border situation at all. The last two weeks has been shambolic and I am making it very clear that a shambolic internal border and a strategy that we stay completely closed to everybody for the next 12 to 18 months is untenable.
"This Government needs to be clear with New Zealand about what that criteria would look like."
Ardern said work had to be done to change flights which were in transit and the way flights were handled before a trans-Tasman bubble could be considered.
"I wouldn't want to put a date on it but it's fair to say there's still things to be done."
Ardern said it was "untenable to consider the idea of opening up New Zealand's border to COvid-19".
