The tiler says my bathroom needs waterproofing again. Is that real?

This sounds like an upsell more often than it is one. The reason comes down to where the membrane physically sits: underneath the screed and tiles. There is no way to look at it, test it or renew it without taking the tiles up, so once a job involves lifting tiles in a wet area, the membrane question is unavoidable.

Signs of a genuine failure

• Damp, staining or bubbling paint on the other side of a shower wall, or in the room below an upstairs bathroom.
• Drummy tiles, tiles that sound hollow when tapped, particularly around the shower base and floor waste.
• Efflorescence, the white crystalline deposit that appears where water has been moving through the substrate.
• A persistent musty smell that cleaning does not resolve.
• Timber floors near the bathroom cupping or lifting.

When it may not be needed

If the tiles are sound, there is no evidence of moisture escaping the wet area, and the work is purely cosmetic, retiling over an intact membrane can be legitimate. The honest test is what is driving the job. Cosmetic dislike of the tiles is different from water appearing somewhere it should not.

What to ask for

Waterproofing of wet areas is governed by the National Construction Code and the referenced Australian Standard for waterproofing of domestic wet areas. Ask for photographs of the substrate once the tiles are off and before the new membrane goes down, and ask who is doing the waterproofing and what they hold, in several states waterproofing is a licensed or registered activity in its own right, separate from tiling.