I am not so sure your Strata manager is correct. Levys aren’t income for taxation purposes but they may be included as part of the turnover that would put you over the GST threshold.
Moving from paying the GST (as your Strata does at the moment) and being required to deduct and remit GST to the tax office is a large administrative burden.
Get a opinion in writing from the Tax Office it won’t cost you a cent and then you have it in writing. The letter should just ask the question:
Is our Owner’s Corporation liable to register and pay GST? If so on what amounts?
Set out the facts ie amount of levys both admin and sinking, interest income and any other income eg rent from leasing common property.