6 Months free Presto - Instructions here: https://www.presto.com.au/telstra
Code reported to be working for current and new accounts.
Code courtesty of @phenomenomenom and @melmac77 (details here)
Update for Virgin users pointed out below thanks to @spackbace + @trump3
Also Virgin subscribers can get free 2GB Data when you subscribe to Presto (remember to check eligibility first)
Update for existing users
Login and go to Settings. Under account Settings it will have your active subscription. Down the bottom is an option to add a promo code.