New Zealand's "hard and early" response to the first sign of the Delta variant of Covid-19 in the community has once again turned heads around the world , but a look across the ditch shows why the decision was made.

On Tuesday evening, Prime Minister Jacinda Ardern confirmed New Zealand would go into an Alert Level 4 lockdown nationwide after a community case of Covid-19 was found in Auckland.

The lockdown made international news with the BBC headlining their story of the situation "New Zealand enters nationwide lockdown over one Covid case".

However, experts are praising the firm response and some are even seeing the aftermath of what happens when the highly-contagious strand of Covid-19 isn't taken seriously enough.

University of New South Wales Professor Mary-Luoise McLaws told Breakfast on Wednesday Ardern’s approach to Delta was the correct one after watching it sweep her state.

"I just wish that we had some of the lessons we could have learnt from New Zealand. Your prime minister is absolutely spot on. We had all the warnings [in Australia], but we did not seem to heed them," McLaws said.

NSW's' Delta disaster timeline

It's been more than 50 days since large parts of New South Wales went into lockdown and daily case numbers have now surged past 400, with officials warning they're not sure if the state has reached the peak of its outbreak yet.

So where did it all go wrong?

On June 16, an unvaccinated airport limo driver tested positive for the Delta variant, triggering a list of exposure sites in and around Bondi.

In the following days, the driver's partner along with those he had fleeting contact with also caught the virus, leading the outbreak to stretch beyond Sydney's CBD.

Days later on June 22, New Zealand paused the MIQ-free travel bubble with Australia and the following day, New South Wales Premier Gladys Berejiklian put her state in a "soft lockdown".

Three days later, restrictions were tightened further but the effort appeared to have come too late with case numbers in the state surging despite the lockdown.

McLaws said New South Wales' situation was made worse by people who haven't followed lockdown rules.

"I would please ask New Zealanders to stay home. Do not break any rules. Do not visit family. This is a highly-contagious virus."

Level 4 NZ's best choice — expert

With such a close case study of what can go wrong with the Delta variant nearby, coronavirus modelling expert Shaun Hendy told Seven Sharp on Tuesday evening New Zealand had already taken the first steps to ensuring it wouldn't happen again.

"I don’t think we’ll find ourselves in the position of NSW, we think that Alert Level 4 will be effective against this variant," Hendy said.

"The way to deal with Delta I think is the way that the Government has chosen to do it, which is to go for the strongest restrictions you’ve got, as early as possible."

Under Alert Level 4, people are asked to stay home and limit their movements unless they are essential workers.

No gatherings are allowed and all public venues, as well as schools, will be made to close.

Businesses like pharmacies, clinics and petrol stations can stay open while all non-essential places must close.

Ardern said financial support will become available for businesses affected.