The Kiwis are focused on getting their Rugby League World Cup campaign back on track in Saturday's quarterfinal against Fiji, but halfback Shaun Johnson admits it will be a failure if they don't progress to the tournament final.

New Zealand's historic 28-22 defeat to Tonga in Hamilton on Saturday means they now need to beat Fiji in Wellington to advance to an expected semifinal against reigning champions Australia in Brisbane.

After leading 16-2 at halftime, Johnson admitted the Kiwis blew their chance to remain on the easier side of the draw, with Tonga's victory confirming them a quarterfinal against Lebanon in Christchurch and putting them in line to play a semifinal against England in Auckland.

"Having a handy lead at halftime and coming out and letting that slip away, we probably blew it," said Johnson. "But then you look at the way (Tonga) played and the energy they played with and you've got to give them credit.

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"We shot ourselves in the foot a few times there with us being a bit too flustered at times and looking to overplay and not just getting into the work that we'd spoken about all week, so it's very frustrating."

The Kiwis are wary of an unbeaten Fiji who top Pool D after downing Italy 38-10 on Friday, but Johnson said it would be a huge disappointment if New Zealand failed to go on to book a spot in the tournament decider.

"I haven't thought that far ahead. With what I believe we can achieve, yes (it would be a failure), but we can't look past next weekend.

"We've got to take a different road now but we're up for it."

Coach David Kidwell said defence and discipline needed to improve to limit the chances given to the likes of Fijian five-eighth Jarryd Hayne, fullback Kevin Naiqama and wing Suliasi Vunivalu.

"They can score from anywhere and have a very strong back five, and they're in form, we have to make sure we're on our defensive game," Kidwell said.

"There are some areas we need to improve on and (the players) were honest about those areas in the sheds and with the little things that make up the big picture.

"Kick pressure at the right times, making sure we complete our sets and get to our kicks, so we can get back into the grind of the game."

Meanwhile, Kiwis centre Dean Whare has returned to Australia for his uncle's funeral and will rejoin the side in Wellington tomorrow, while wing Dallin Watene-Zelezniak is in doubt for the weekend with a knee injury.