Normally Optus does $10 per ticket, but until 11th January you get a free child's ticket as well.
Details at:
https://payment.optusmovierewards.com.au/Faqs (requires login prior)
There is a specified limit of 10 tickets(likely 20 including the kids' tickets) per 24 hours.
Purchasing using the email delivery option and you'll receive the code (to use at the Hoyts website) within 10 minutes or so.
If I buy an email voucher, is it sent straight away?