Jimmy, don’t hang your hat upon the the fact that the Development Approval that issued when the building stated that the approval was for the building of residential home units. Town Planning zones change over time, usually from low density to medium density to high density. It is the current zoning that is relevant.
Having said that, a residential zoning will be no protection against the new short-term rental accommodation policy because the government is proposing a ‘Short-term Rental Accommodation Planning Framework’ under which it will implement a state-wide planning framework which will introduce in all residential zones the ability to use a property for short-term rental accommodation without Council consent (called an exempt development) provided the property meets minimum fire safety and evacuation requirements.
For more click https://www.planning.nsw.gov.au/policy-and-legislation/under-review-and-new-policy-and-legislation/short-term-holiday-letting