They actually come on in the afternoon can't remember if it's 4PM or 5PM Sydney time.
For example right now as I type this, the furthest date I can select in the calendar when booking is April 13th 2017. When it returns results it will say April 14th onwards is unavailable.
However, after the trigger time (3/4PM), the calendar will still show April 13th 2017. However, the results will show availability for April 14th. I've actually noticed you can sometimes book a lot of reward seats (this is the website bug I hope Virgin never fix!). You can also book tickets too, though sometimes it's a premium until the calendar updates.
When the calendar updates, results often differ.
... I think the front calendar updates around midnight.