The 19-year-old raised her arms in celebration after soaring over 4.60m, the mark for direct entry into Friday's final.
But she'd struggled early on in qualifying, needing three attempts to clear her first height at 4.45m and two attempts to make 4.55m.
McCartney made no mistakes at 4.60m, however, soaring over on her first attempt to become one of seven vaulters to reach the automatic qualifying height.
Afterwards, she said it had taken her a little longer than usual to sort out her pole selection.
"I think nerves were a little bit of it, but what really happened was that I was blowing through poles.
"I was making them too soft, so I kept hitting the bar when I was on the way up," she told NZ Newswire.
"That just means I need to get on a bigger pole. I was still matching the pole to the height at that point.
"It's not a bad thing, but you still want to clear the bar."
McCartney isn't putting any limits on what she might achieve in the final, a wise move given her rapid improvement over the last two years.
She has a best of 4.80m set in March this year, and has improved 35cm in the last two years.
However, she also has limited international experience - her first major competition was only five months ago at the world indoor championships, where she cleared 4.70m to finish fifth.
With qualifying safely negotiated, McCartney is now looking forward to sitting down with coach Jeremy McColl to begin planning for the final.
"We know now what poles I need to be using, so that's a bonus," she said.
"The final is a very strategic event. People will be passing heights at all sorts of places so we'll have to have our game plan ready."
Among those expected to feature are defending Olympic champion Jennifer Suhr from the US, who has cleared 5.03m this season, and Greece's Ekaterini Stefanidi, who cleared 4.60m with the only competition jump she made.
