You need at least a 3 in. pipe -- shower is 2 in. pipe
2"
Water won't drain from shower. Either trap or shower drain line is clogged.
Normally 2"
No. Australian pipe threads are metric and the US is not.
the shower and bath water comes from two different places so their is two different pipes. Get the shower pipe checked out by a plummer and they cause fix the pipe.
A poor plumbing job. The pipes to the shower & sink share a pipe to the water heater and the pipe is not large enough. This causes the water pressure in the pipe to drop when the hot water is turned on at the sink while the shower is also on.
This question is vague as it doesn't state where exactly between the main and shower head it has broken. However, worse case scenario, the wall will have to be excavated a little to allow for the pipe to be replaced, using the required fittings to go with it.
Shower drain goes into a trap under shower. Continue piping from trap to drain line.
Incorrect pipe size installations
It all depends how it was attached in the first place. Most are fitted to the shower pipe where it comes through the wall. There are attachments and clips to do this. My shower pipe has a TEE fitting on the end with one leg going to an overhead shower and the other to the handheld. There are many variations of this. Cruise your plumbing aisle of HD or Walmart and you will find lots.
Yes, plastic pipe, PEX, can replace copper anywhere now.