Well, first things first, if Pohatu were supposed to be australian, he would sound a lot like the standard LEGO australian accent, such as Nex or Surge (IFB) from Hero Factory. However, since it’s all one guy, it could be the case that he actually can’t do an australian accent. Except, when you compare the clip to someone like Ray Winstone who is not australian, It sounds very similar. But when you compare it to someone who IS australian, then you can clearly see Pohatu is not australian. One last thing to add, which is very important, Australians will always add an upward inflection to the end of words and sentences. No where in the clip does Pohatu use upward inflections, instead he uses downwards inflections, which is most commonly found in an english accent.
Now, of course, the boomerangs and the desert theme kinda hint he would be australian, but from the evidence, this is either a really bad impression of an australian or he is english. I will say this, some people (mostly americans) do confuse an english accent with an australian accent. When I was in New York back in 2012, I obviously had some encounters with americans. Now, my accent is a standard english, surrey (almost identical to Viper’s accent). Now as you know, Viper is not australian. So, when I spoke to them, the first thing they said was “Are you australian?” to which I replied “No, i’m english.”.
So, from this evidence, I’m pretty sure this is an english accent. Plus, as I said, I’m english myself! I go into london on a regular basis and they all sound like Pohatu!