Queenstown draws heavily from both the New Zealand and Australian domestic travel markets, and school holiday periods in either country create a noticeable, predictable spike on top of whatever the underlying season is already doing. December–January is the extreme case, since it overlaps with the long summer break in both countries at once, but shorter school holiday windows through the rest of the year also produce smaller, more localised bumps.
What actually changes
The visible effects are the same ones that come with any peak: accommodation availability tightens first, followed by popular activity time slots, with restaurant wait times and general foot traffic in the CBD following close behind. None of this makes Queenstown a bad choice during these weeks — it just means booking earlier and expecting a livelier, more family-oriented atmosphere than during term time.
Why this is worth checking specifically
Because New Zealand and Australian school terms don't always align exactly with each other or with the Northern Hemisphere's summer break, it's easy to assume a date is "off-peak" when it's actually inside one country's holiday window. If your trip dates are flexible, checking both countries' current school term calendars before booking is a quick way to sidestep a crowd spike you weren't expecting.