Bagot,
From a Jetstar promo email:
Singapore-Auckland flights are operated by Jetstar Airways (JQ) for Jetstar Asia (3K).
The SIN-AKL-SIN flights operate with a 3K flight number but the airframes used are VH- registered A330s.
That is, Jetstar Asia (3K) use their SIN AOC but charter the capacity from JQ (Aus).