You have answered your own question – if there is not change of use, why do they require a DA?
The establishment of the change of use can be found in the result of the Land & Environment Court findings with regard to City of Sydney’s case against Australian Executive Apartments and its operations in the Bridgeport building.
The LEC found that short-term lets could not be conducted under the terms of the Residential Tenancies Act and therefore were illegal because the building was zoned residential only.
Every aspect of a building’s operations is related to common property, including access, lifts, electrical supplies, water provision, sewage, lighting and maintenance. Short-term letting puts extra strain on many of these resources.
Also, there are special requirements for holiday lets, such as emergency exit signage and parking, all of which affect common property.
Go to the meeting and if the motion is not a special resolution, tell the chairman you will challenge any affirmative result both at council and at NCAT. I would also ask him to resign as he is leading the building into areas that will cost you all a lot of heartache (and possibly money) without checking his facts.
By the way, what does your council’s DA form say about strata applications? Is it the same as City of Sydney?