remove plywood and replace with hardiebacker?

[shardy]

I've attached a picture of our new loft bathroom space where the shower cubicle will sit.
Its on top of an RSJ so I think the floor will be solid.
I was expecting to use hardiebacker on the shower area, but the builders have used ply.
I think it might be normal ply as it appears to be 5 ply thick. I've read that marine ply is made of approx 9 thinner plys.
If it is normal ply should we waterproof it with something, or take out and replace with hardiebacker or marine ply?
We will tile a bit past the shower area so would like whatever we use to stay flush with surrounding plasterboard.

 

[shardy]

tiler had a look and says the ply is fine. once its plastered, unibond over it, that's it. sounds good to me. plumber will put shower tray in first on a cement bed