It wasn't super difficult, and was quite fun. I'm not a cryptography person and have zero knowledge of ciphers or anything like this, but a friend and I saw it and thought it'd be fun to give it a shot. We shared ideas but pretty much solved each puzzle ourselves in different ways (which was interesting to see in itself).

The hidden 5th puzzle was both the hardest to get going on (due to no hints compared to the others), while also being among the easiest once you figured out what it actually was.

I did a little write-up here if anyone's interested: http://senwerks.com/hacktheplanet/Solving-the-Australian-Sig...