Her first stumble was at 3.50m, but Chan got over with only a slight rattle on her second attempt, which meant she was trying for a new personal best at 3.70m, hoping to exceed her own 3.55m mark.
After Chan was unsuccessful in her three tries, McCartney began her own session at 3.90m.
There was a little slip at first when she cleared the mark but her pole came back and knocked the bar off, but the world-class vaulter made the necessary adjustment to be well over on her next try.
She also cleared 4.10m on the first attempt, but came unstuck on her three tries at 4.20m.
The best chance of the three attempts was her second vault, where McCartney managed to clear the bar but her hand made contact on the way down.
Just how much the conditions were affecting the vaulters' chances was clear in the men's division as neither New Zealand schools record holder Pascal Kethers or visiting Australian senior men's competitor Maxim Mischenko could beat the 5.00m mark to match their 5.10m personal bests.
In addition, German athlete Vince Hobbie had made a 4.70m clearance in the Hawke's Bay only the week before, but dropped out with three failed attempts at the 460m mark.
All three men would pull out halfway up or slipped when trying to secure their takeoff at various stages of the session as the rain got heavier.
Both Kethers and Mischenko both comfortably cleared 4.60m, with the Australian also getting 4.80m on his first attempt while the Under 20 Kiwi missed his first two efforts but nailed the third under pressure.
Mischenko had the advantage at 5m if the event went to countback, but both men were more determined to exceed their own best efforts rather than beat each other.
In the end, neither could advance any further, as Kethers came close on his first try but pulled out on his next and then slipped without leaping on his third.
Mischenko went the other way, not completing his first two vaults but getting up and just missing on the way over on his final try.
In other events from the afternoon session, Bay Cougars' Jenni Scott was the best of the women in the triple jump, with the North Shore competitor saving her best till last with an 11.72m effort after the rest of the field had been eliminated.
Hawera's Arnold Fage had only himself to compete with in the men's section, but improved as he went on and finished with a 12.15m jump on his final attempt.
The evening session began after deadline.