While Hales (19th) and Rayner (23rd) finished out of the running in their second runs, the semifinal onslaught was just getting started for the Kiwis.
Another 16-year-old, Hannah Thomas, needed only one run to make it through to the under-18 C1 semifinals, comfortably qualifying 12th-fastest despite picking up a 2s penalty, with a 115.32s time.
"There was an awesome atmosphere from Team NZ on the riverbank and that was a big help," Thomas said.
"The top part of my run was awesome - I was feeling confident and strong on the water but I did have to go back around on Gate 15, which cost me a lot of time, then came low into Gate 17. Apart from that, I put down a solid run."
Rayner, who had a busy day, was 34th in her first run and 19th in her second run.
Rotorua's Zack Mutton then picked up where his younger sister River had left off, posting the 12th-fastest time in the under-18 men's K1. His 85.23s time was just 3.80s behind leading Frenchman Anatole Delassus and nearly 6s inside the top-30 who automatically progressed.
His 15-year-old Rotorua teammate George Snook, the youngest member of the New Zealand contingent, posted a 98.29s time to be 51st after the first run, while Damian Torwick had a nightmare first run to lie 61st.
Tauranga's Torwick, 17, turned things around completely in the repechage run, however. He blazed down the course clean, posting the fastest time of the second run to storm into the semifinals. His 84.96se time would've been good enough for a top-10 finish in the first run.
The under-18 men's C1 paddlers couldn't repeat the semifinal success, although Tauranga's Stewart Bloor came closest. He posted a clean 102.99s first run but got stuck in a move just above the finish line, which cost him valuable time and left him in 30th.
Jack Egan was two spots further back, after a 104.01s effort, while Charlie Bell was 44th another 3s back.
Bloor picked up a touch in his second run to finish 12th, just two spots outside the semifinal qualifiers, with Egan 16th and Bell knocking 14s off his first-run time to finish 17th.