Optus put all sorts of specials up through the year however they do always seem to insist on really low data amounts once you drop below a certain spend per month. However you should do the maths on the plans as Optus is already offering what you want.
The $10 Epic Plan covers 21 days, so you could buy it for a year at a total cost of $175 and get around 8Gb of data.
However grab the $180 Epic plan (which lasts a year) and that nets you 60Gb for use (plus its set and forget)
So for $5 more a year, you get 5Gb a month to use instead of 0.5Gb a month.