Removing bath instal Hobless shower

First up - I'M NOT A PLUMBER - my knowledge is limited to three bathroom renovations - so take my advice with a grain of salt.

The waste pipe should be cut off at a height level with the top of the tile base (the level you indicated by "concreted with a slight slope", in fact it will be just sand and cement). I do it before I lay the tile base- this gives me a height to work to when shaping and levelling the tile base. Of course this means you have to estimate the projected height of the tile base. If you are not confident of doing that, then you can cut it off after - I just think it gets in the way when you are trying to float off the tile base. In any case, if it is cut to height before waterproofing then you can get a good seal around it by letting the waterproofing go into the pipe a bit.

The metal grate that you will be putting in over the waste pipe will have a lip on the inside that should fit neatly into the waste pipe. The grate should be just a bit thinner in height then the tiles - so water will fall off the surrounding tiles onto the grate. You just fix it on at the same time as you are doing the tiles.

I have to say,though, that I would be looking at blowing away the 50mm pipe and replacing with a 100mm waste. My reasons for this are:
1. I think 100mm is the published standard in NSW. You should be building to the standard, though I know it is hard when doing a renno. Is there a plumber out there who can confirm this ???
2. It is unlikely the bath waste was exactly in the middle of the area that will be the new shower floor. Its just a personal thing - but I think wastes look better in the middle, and if not quite in the middle they look daggy. Also, if usuing large tiles it will be hard to get a good lie if the waste is biased to one edge.
3. A 50mm waste in a shower just looks stingy - like what you expect to see in caravan, not a house - (again, personal taste).

Also, have you considered whether there is trap or gulley pipe in the line. If the bath had its own trap attached, then there probably wont be one on the line so if you dont put one in yourself then your shower waste will just be on a straight pipe to the sewer - very smelly and illegal too.

If your house is 30 or so years old or older then you probably have vitreous clay pipes. If so, it is probably not a big job to replace with 100mm. As you are in Sydney, there should be a VCP pointing upwards immediately under the old bath waste - with its collar just below the surface. Chip down to it, clear around, and fit the 100mm with cement, a plumbquick or a vcp-pvc connector, whatever the standard allows. Unfortunately there have been a lot of different ways of plumbing up bathrooms so it is difficult to say anything with certainty - I'm just going on the rennos I have done. Moving the waste to a new location and/or fitting a trap is a bigger job - difficult if you dont have access below the slab and have to cut down from above - I know as I have just had to move all the wastes in my bathroom - using a concrete saw and a lot of effort.

If putting in a new trap and waste pipe, there are quite a few things to get right - for example inclination (10mm in 1mtr, I think), height the weir of the trap should be above the outlet pipe invert, height of weir below top of slab etc. It will take a bit of study first.

To answer your original question - I notice that 50mm for shower outlets is within the standard in parts of Europe and the USA - so it must be able to carry the water - unless they have weaker showers or something.

hope this helps
Arron