Stumbled across this when looking at switching to another NBN provider. I believe this is only available if you log into your account and select Change Broadband plan under 'My Services'.
On their website, it lists Superfast 250/25 (200Mbps Typical Evening Speed) offer $99.99/month for 6 months then $129.99/month
Tried uploading my snip of offer - uploaded it here: https://ibb.co/vYvSsdb