Dear Fromthenorth
You don’t actually tell us what your problem is, apart from the fact that some owner has the temerity to fo some work around your building.
There are only two trades, electrical POWER wiring and telecommunication wiring, and plumbing including gas, which require a licence. All other maintenance trades do not require a licence, though the trade may need to be registered with fair trading.
If the lot owner doing the work is suitably qualified, then let them do the work. They probably are cheaper than a tradie and can do it sooner. I can’t see that therewoild be any issue wiyh insurance or warranty unless they were proved to be negligent (but that argument applies to any trade doing work onyour property).
If there is an owner who is willing to do some work around your property, he is probably saving your strata money.
Having said the above, this arrangement probably works well for smaller complexes.