Those ratings are pretty average honestly. For premieres in Australia you're looking to get over the magic million mark (both The Block and 7's Zumbo's desserts both managed to do that). So in that sense, it's not doing too well.
I wouldn't worry though. There has been mass hype for Survivor in the lead up to the premiere and feedback from the public has been positive. Zumbo's show was apparently trash, so expect that to die off in the next few weeks. It's main competition will be The Block, probably why they've added in the Tuesday show to go head to head with it.
Gamer's right, if they thought we were doing shit it'd be dumped to a later timeslot, not given an extra episode at 7.30.