As settlersbill has said, they do have the Very Fast (the 50/20) and Super Fast (the 100/40) Speed Boosts... but you would currently be getting about what would be expected from the standard tier.
As far as why, Telstra set the HFC cable and Velocity speeds themselves so they set their standard speed at 30/1 having decided that obviously that was a good speed for them to set, whereas with the NBN the speed tiers are locked in by NBN Co, meaning Telstra would have to provide one of the 5 specific tiers, and the 25/5Mbps was the closest to that speed they were aiming for on the other technologies...
So even if Telstra wanted, they couldn't actually give you 35Mbps download on NBN, that being said if you were provided a guarantee that you would not lose speed, about the best recommendation I could make would be to call and see if they would be willing/able to give you the standard 50/20 speeds.
As far as why, Telstra set the HFC cable and Velocity speeds themselves so they set their standard speed at 30/1 having decided that obviously that was a good speed for them to set, whereas with the NBN the speed tiers are locked in by NBN Co, meaning Telstra would have to provide one of the 5 specific tiers, and the 25/5Mbps was the closest to that speed they were aiming for on the other technologies...
So even if Telstra wanted, they couldn't actually give you 35Mbps download on NBN, that being said if you were provided a guarantee that you would not lose speed, about the best recommendation I could make would be to call and see if they would be willing/able to give you the standard 50/20 speeds.