On Thursday last week New Zealand's borders were closed to all foreign arrivals.

Sir John acknowledged she has been criticised for this, with people saying it came too late.

"I know from being in that job that while maybe one person's perspective, you have to weigh up a lot of different things," he said.

"So to say close the border like she did, on one hand that will save lives but that's an immediate recession which affects the most vulnerable who are the least equipped to deal with it.

"Those are big calls for a Prime Minister and I don't wanna say it's lonely, but the buck stops with the person at the top," he said.

The former Prime Minister also said he was glad to see Ardern being tough on rule breakers.

"I do think you'll see quite high levels of anger over time when people start flouting the rules and I think 'good on Jacinda' when I see her telling people off for that," he said.

He refused to criticise Ardern saying that not only would it be unjustified, but it's not what the country needs.

"This is a time we all have to pull together on this. This isn't a political issue - it's a catastrophic health emergency that the whole world is facing."