I asked the specific question of AirNZ a couple of days ago and this is how it went -

Me "Please provide the full terms of the credit, how it can be used and any restrictions?"

AirNZ "The full amount is held against each booking. When re-booking flights it must be all used at once of equal or greater value. The credit must be used before X"

That was the extent of the reply so from that I take as long as you spend the credit held in one transaction then anything goes Intl long haul, TT, domestic. But the catch is if you are sitting on thousands it might be hard to spend on domestic & TT in one booking.