Cairns placed fourth in the women's heat, but was the first Pacific nation to cross the line which was all she needed to book her place.
Cairns' coach, Gavin Elmiger, who watched the results live from Auckland said he was ecstatic with Cairns results.
"She didn't realise initially until they said 'hey look you've qualified' and she was like 'what?' She couldn't believe it, so now there's a possibility that one of the Cook Island girls may qualify for the games as well, so it's exciting."
Elmiger said Cairns was the perfect role model for anyone wanting to achieve an Olympic dream and that she was opening doors for other Pacific athletes who might want to take up the sport.
"These girls put so much in. They haven't got the support of those massive big countries and then now they have the opportunity to go onto an Olympic stage and represent their country," he said.
"This is what these continental champs is all about, getting the smaller countries on to the World stage so that they can then go to the Olympics and represent their Pacific Island Nations and all their countries really."
Cairns can relax despite having one more race in the doubles heat with her sister Bridget on Saturday but Elmiger said it's all about the Olympics once she returns home to New Zealand.
"Her focus will be on the Olympics, so trying to get to her best result when she gets back. We'll probably have a bit of a debrief but then we'll be looking at getting her best performance for the Olympics."
Meanwhile in the men's races, Samoa's hopes to qualify are still high despite Rudolph Williams and Clifton Tuva'a finishing fifth in their doubles heat.
Elmiger said the strict qualifying times put in place for Australia and New Zealand means there is still a chance they [Williams and Tuva'a] could be heading to Tokyo.
"There's a strong possibility that could be the case for both of them so that means both of them could be going but we won't know that for the next four or five weeks," he said.
And despite suffering an injury just over a week out from the event, Tonga's Pita Taufatafoa will be chasing qualification for his third unrelated sport title for the Kingdom when he races in the K1-200m sprint later on Saturday.