The $131.50 liability currently showing in myUNSW refers to the student amenities fee for semester 1
You need to apply for SA-HELP (further information in myUNSW) if you want to defer this payment. The SA-HELP form is online and available under 'Commonwealth Assistance Forms' on the 'My student profile' tab within myUNSW.
HECS only covers your tuition contribution fees.
THe deadline for applying for SA-HELP is the census date (i think) in the first week of March.