A small crowd, a quiet Christchurch day in the last week of February, an Australian rise to No.1. It could have been Allan Border overtaking Sunil Gavaskar 23 years ago to become the leading run scorer in Tests. Instead it was Steven Smith's men nudging ahead of India to become the top-ranked Test side in the world. For Border, it was the culmination of 14 years as a Test batsman; for Smith's team it feels like the result of four good months.

In fact, it is the culmination of four years of cricket, under not only Smith but his predecessor Michael Clarke. The current rankings stretch back to 2012, so they include Australia's home Ashes whitewash in 2013-14 and their series win in South Africa that followed. After those triumphs Australia briefly jumped to No.1 under Clarke but that lasted a matter of months, and their task now is to not only hold their lead but extend it.

And they must do so without Clarke, Brad Haddin, Shane Watson, Mitchell Johnson, Chris Rogers and Ryan Harris, all of whom retired over the course of 2015. Those departures have left Australia with a significantly different looking side under Smith, and while they have started well by winning at home to New Zealand and West Indies and now away in New Zealand, other challenges await.

They include a series in Sri Lanka later this year and then home Tests against South Africa and Pakistan, followed by arguably world cricket's toughest challenge: an away Test series in India. How long Australia can hold the No.1 position remains to be seen, but under Smith and vice-captain David Warner, the initial signs have been good.

"A lot of credit has got to be given to the guys who have recently retired as well," Smith said after winning the second Test at Hagley Oval. "The ranking system goes for quite a while and those guys were quite a big part of our success as an Australian team. Having said that, the guys that have stepped up to the plate since those six big retirements have been outstanding and I'm really proud of the way the boys have been performing."

Such has been the success of Australia this summer that all members of their top five are averaging 50-plus since Smith officially succeeded Clarke as captain. The stars have arguably been Adam Voges and Usman Khawaja, but in the second Test against New Zealand it was Joe Burns who stepped up, earning his first Man-of-the-Match award for his 170 and 65.

The value of his patient first-innings century should not be underestimated, given that New Zealand had made such a remarkable start to the Test through Brendon McCullum's record 54-ball century on the first day. New Zealand reached 370 in their first innings, and it was the first time under Smith that the Australians had needed to fight back from being behind in the match in the early stages.

"We did have to fight," Smith said. "I thought that probably on the first day somewhere around 250-300 would probably be par on that wicket and they really took it away from us in that second session. That's a credit to the way that both Brendon and Corey Anderson played.

"They came out and really took the game on and we didn't know what to do for a period of time there. But, credit to the guys the way we were able to fight back scoring 500 in our first innings again, I think that really sets the game up for you and I thought the guys responded really well."

Smith was also key to the turnaround, accompanying Burns for much of the second innings and registering his third century of the Test summer. Although Smith was charged with a Code of Conduct breach for his remonstration with umpire Ranmore Martinesz on the fourth day at Hagley Oval, the retiring McCullum said he believed that under Smith the Australians focused on "positive play" rather than verbals.

"I think the team plays slightly differently to what they have done previously," McCullum said. "Most Australian teams play the way their leader is. If you look at Allan Border, Steve Waugh, Ricky Ponting, Michael Clarke, they always take on a slight twist. Under Steve Smith, we've seen a very similar thing.

"This series has been played in great spirits, I think, and the one back in Australia. I think Steve Smith has been a catalyst for that. He plays the game for the right reasons as well. He plays with his heart on his sleeve but he's a very respectful guy and a wonderful cricketer. They're No.1 in the world now and a lot of that is to do with his leadership and Darren Lehmann's as well.

"They probably skin the cat slightly differently. The majority of their focus is on positive play rather than necessarily some of the semantics of past eras. They go about their work in a nice manner and they play hard cricket but in the series that we've had, I don't think they've ever looked to step over the line."

As a result of the victory in Christchurch, Smith remains undefeated as Test captain, with seven wins and four draws from his 11 games in charge - including when he was standing in for the injured Clarke in 2014-15. Under Smith, Australia have beaten New Zealand in four of five Tests this summer, and McCullum said Australia had the potential to remain at No.1 for some time.

"I think so because they've got depth as we've seen in these two series," McCullum said. "The bowling line-up in this series is quite different and they've all stepped up and that's the beauty of Australian cricket. They've got so much depth that if they can get everyone heading in the same direction and their game-plans are simple, then they're going to be dangerous no matter what.

"That's why they've been able to rise to No. 1 in the world. They've also got some outstanding batters, not necessarily the flashiest batters that Australia has produced but they're churning out runs and when you do that your bowling attack becomes a lot more potent."

Brydon Coverdale is an assistant editor at ESPNcricinfo. @brydoncoverdale