Speaking from experience, they tend to act in self interest and simply want to get something out of it, rather than what is best for the strata.
Just to add to JimmyT’s comments.
If you do choose to go ahead with an onsite manager, make sure that you also look at insurance, statement of duties, how they will be paid (wages or contract). This adds all sorts of legal & insurance implications. It also costs your time, other committee members time and strata manager costs.
Also any contract to be done up independently of this person so no bias. Also the invoices they send you must be proper invoices, conforming to ATO standards. Also make sure you are able to contact this person and hold them account of there work.
Also you may want to look at a competitive tender process. That way you can honestly say this is the best person for the position. Get a proper signed contract then you can hold them account for their actions.