Yes, well done for taking it on, particularly without a hand-over from the previous EC. Much is communicated verbally as new people join the EC and old people leave and ideally some stay on through the transitions.
GST can be confusing. Our budget is all set out on the basis of the costs anticipated without GST, and the same for our levies. Then GST is added to our levies when individual owners are billed. Similarly, there is GST to be paid on most OC costs. The GST collected on levies has to be passed on to the ATO but the GST we have paid on our costs can be deducted from that. The consequence is effectively that the GST that is passed on to the ATO is approximately zero.
Your strata manager should be able to explain that to you. The EC is relying on the strata manager to get the accounting right. If you can’t satisfy yourself that it looks to be correct and reasonable then keep asking questions. Remember their job is to satisfy you that the place is being managed properly.
In the specific instance, the strata manager should be able to provide a copy of the invoices from the building manager and it they don’t show whether there is GST included or not, then you have to ask how the SM is doing the accounting correctly.