Erika Fairweather took off her goggles and shook her head in disbelief, having just booked a shot at history.

The 17-year-old Kiwi last night swam the race of her young life in the 400m freestyle heats in Tokyo, qualifying with the fourth-fastest time for this afternoon's Olympic final.

Fairweather set a new national record with a rapid effort from lane eight at the Olympic Aquatics Centre, stopping the clock at 4:02.28 to best the previous mark set by Lauren Boyle at London 2012 by more than a second.

In today's final the Dunedin high school student will face off with five-time Olympic champion Katie Ledecky, seeking to become New Zealand's first swimming medallist since 1996.

"It's unreal, it's so much faster than I thought I was going to go," Fairweather told Sky. "I was so focused, I was like, 'I'm not going to stuff this up, the opportunity's here and I'm going to grab it'. And I did."

Fairweather did just that by flying through the first half of her heat, sitting second behind world No 1 Ariarne Titmus of Australia at the 200m mark. The Kiwi had fallen to third with 100m to race but showed composure beyond her years to power home and touch 0.62s behind Titmus.

Moments after taking more than four seconds off her personal best, the cameras caught Fairweather looking up at the scoreboard in shock.

"I was like, 'Is that me? Are you sure?'" she said. "I just have no words right now."

The teenager's speechlessness was understandable given her time was the fourth fastest in the field, trailing only American great Ledecky, Li Bingjie of China and the top-ranked Titmus.

Fairweather now faces a busy day. After the final - set to begin at 2.20pm - she will be back in the pool for the 200m freestyle heats at 10.05pm.

The shorter distance is supposed to be Fairweather's preferred event, having won a 200m junior world title in 2019. But it might not retain that status for long.

Listen to Tokyo in 20, the new daily podcast from Newstalk ZB, looking back at the day's action from Tokyo 2020 and previewing what's coming up for our Kiwi athletes.

Newstalk ZB - Official Radio Broadcast Partner of the Olympic Games Tokyo 2020.