Hi - The first thing I do is check the postcode against the suburb given.You will normally get a list. If that's correct then I check the name of the suburb- eg West Melton - should be re-entered as Melton West.

Aus Post likes the NSWE directions after the area.

If that doesn't work I then put in a suburb matching the postcode that does work - then manually change it on the label when it's printed.

Try those and see if they work.

(Although you've probably fixed it by now)