I suspect you'll be able to select a seat upon check-in. Not 100% sure on this, however.

If you want to reserve a seat before then, you can try calling ANZ. There's a good chance they'll just tell you to contact SQ, however.

One other thing you could try is to manage your booking online on the ANZ website. If their website recognises your booking reference, it may allow you to select a seat. You might have to try different booking references associated with your ticket for this to work (assuming you can find them).