A bath overflow is that small round hole near the top edge of the tub, usually connected to the waste pipe. If you accidentally overfill your bath (or get distracted mid-run), it helps redirect excess water down the drain, instead of letting it spill over the edge and turn your bathroom into a shallow swimming pool.

If you have a floor waste in your bathroom, an overflow in your bath is optional. If you do not have a floor waste in your bathroom then you require an overflow in your bath.
According to the National Construction Code (NCC), baths do fall under "wet area" regulations, especially in multi-dwelling buildings like apartments. The code says you need some kind of overflow protection in these cases, either a floor waste in the bathroom, or a built-in overflow on the bath itself.
That means, in many builds (especially newer or multi-level ones), you’ll need an overflow, or at least a plan to deal with overflow water safely.

A few things to keep in mind:

• The overflow drain doesn’t drain super fast — it’s more of a slow leak than a full-on emergency escape hatch, meaning the flow into the bath could still overwhelm the overflow exit, in the case of taps being left on
• They can be fiddly to clean, especially if gunk builds up inside
• Some minimal or designer bath styles skip the overflow for aesthetic reasons — so you’ll need to plan around that (like including a floor waste)

Freestanding baths don’t always come with overflows and installing one after the fact is tricky. If you’re eyeing off a sleek centrepiece bath without an overflow, make sure your bathroom floor has a proper floor waste (and falls correctly to it), or check with your builder if you're ticking all the boxes for compliance.

If you’re:

• Planning not to include a floor waste
• Building or renovating in an apartment building
• Just prone to zoning out during bath time with a glass of wine... then yep, having an overflow is probably a wise move, and in some cases, it’s non-negotiable.