Where to Fill
Many dump stations have a potable water tap (often marked). Council facilities (e.g. some public toilets or camping areas) sometimes have taps — check signage. DOC campsites often have water; not always drinking quality — boil or treat if unsure. Holiday parks usually have water at powered sites. CamperMate and Rankers list water points.
Safety
Only use taps marked potable or drinking water for tank fill. If in doubt, boil or use a filter/UV. Keep your fresh water tank and hoses clean to avoid contamination.