score:7
It is possible, but not all that widely advertised!
When you get to the station, head to one of the ticket gates with a member of staff on it. Ask them if you can go through the underpass to get to the gardens, and they'll give you a visitor pass and let you through. When you get to the gateline at the other side, again head for a staff member, and give them your pass. They'll open the gate and let you out. From there, follow the signs up up up and into the parklands!
Alternately, if you have a Go Card, you can touch in to open one set of gates, then touch out at the other side of the station and you won't be charged.