i purchased property in sep 2017 have not yet payed GST

i purchased a property under construction in sep 2017 my builder is asking 12 % GST
I HAVE NOT YET PAYED GST AS THERE IS STILL OC AWAITING
HOW MUCH GST IS APPLICABLE TO MY PROPERTY.????
PLZ HELP HOW I CAN CLAIM A BENEFIT OF REDUCED GST RATES.????