This is a much more common problem that you would think (and it’s one of the reasons I would never buy a terrace apartment).
But there is a simple solution. A relatively cheap web cam, positioned at the front edge of your terrace and pointing up, recording on to a laptop at, say, one frame a second, would not only identify the culprits pretty quickly but give you evidence to take to NCAT to have them breached and fined.
Cameras and software are cheap (the latter may be free) so all you need is a hand 14-year-old to set it all up for you.
Or, a notice in the lift saying you have installed security cameras pointing up at the balconies might have the deterrent effect you require.
If you can identify the balcony, then a Notice To Comply can be sent to the resident, whether it was them or a visitor, as they are responsible for the behaviour of people they invite into the building.
And before anyone says anything, no this is not an invasion of privacy, providing you are not filming people getting their kit off or doing anything they would otherwise expect to be private.