Don’t know if you need a lawyer we haven’t thus far. Things to remember:
1) You’re under no obligation to provide power for free.
2) The Owner’s Corporation owns the internal building cabling so you’re under no obligation to provide access to it.
3) If you do accept an offer supervise the install so that there will be room for other equipment in the comms rooms/cabinets down the track. i.e. make sure no one company is making a real estate grab for space & excluding others.
As for the NBN your Owner’s Corp may contact them directly & find out when you’re due for an install & if there are any ramifications by accepting another ISPs offer. There is much more info on ISPs issuing LAANs on the whirlpool broadband forums here (search for LAAN, FTTB and/or crosstalk): https://forums.whirlpool.net.au
If you want to read more, Schedule 3 of the Telecommunications Act is here:
https://www.austlii.edu.au/au/legis/cth/consol_act/ta1997214/sch3.html
This blog post from last year may also be of interest:
https://blog.lindsaystrategic.com/2014/04/08/carrier-access-powers/