Hi Waterboy,
I agree with Jimmy, from your floor plan it appears to be an internal problem, and thus for you to fix.
However, I do note it seemed odd to me that the wetness penetration is on the opposite side of the bathroom to that where the tiles were removed. The photos show the shower spray directed at the floor. I am wondering if the water is being introduced into your apartment’s floor slab through the shower floor?
To investigate this you would need to engage a high level waterproofing consultant to conduct methodical testing, likely including dyes. Then, if the problem is found to be in the floor waterproofing and you do claim against the owners corporation it is possible they will apply a sealant to the shower floor. You would save time and money just moving ahead now and doing this work yourself, especially as the apartment is tenanted and you cannot leave your tenant living with water and mould problems.
It may be worth asking your neighbours if anybody else has a similar problem? If bathrooms generally have been poorly waterproofed it is only a matter of time before the apartments below start to show damage.