Qantas Credit Voucher - more travellers than the original booking

My wife has a Qantas Credit Voucher from a single cancelled domestic business trip (ADL-MEL-ADL). The T&Cs state that to be used, the new booking must include “everyone who was on the original booking”. My question is - can it be used as “part payment” for a bigger international trip (ADL-MEL-HKG-MEL-ADL) for the two of us?

Yes. You can use it to cover part of the ticket costs and pay the rest another way.