APhO is very, very different from APMO. APhO's almost as selective as IPhO - the teams are the same (think of physics as having two olympiads that the team members participate in), just slightly larger, and some members opt not to continue onto IPhO due to the immense time commitment.
It's probably better to think of APhO as IPhO, but restricted to the Asia Pacific region - to tell the truth, this probably makes it more competitive! The selection process for APhO is identical to that for IPhO if teams don't enter APhO (which happened last year due to the security situation), so getting in's roughly as difficult as making it into the IPhO team if it were chosen directly after the summer school (people tended to work harder once they knew APhO was going ahead because they thought they had more of a chance!). The exact IPhO team will be confirmed after APhO, but the APhO team's still the team anyway - most of the competitors will go on to IPhO (it's impossible to go to IPhO without doing APhO), and besides, the preparation for APhO and the competition itself are pretty much the same as IPhO.
APMO's very different as well - APMO requires competitors to be in the top 30-40 maths students in the country, whereas APhO is much, much more selective. The training for APhO's identical to the training for IPhO if Australia doesn't send a team to APhO - the team goes to the Easter Training Camp, does a lot of extra work and goes away. We're going to Taipei this year, so that should be fun! APhO runs identically to IPhO - the structure of the programs are the same. The APhO website's here (
http://www.phy.ntnu.edu.tw/apho2010/) if you want to have a look at how it works. I know that the APhO team also receive Australian team shirts (all of the I*O teams do too), and I heard from one of the tutors that we also get the Australian blazers, but I can't confirm that.
As for VCE and science olympiads? That's a bit more complicated. To give you a bit of an idea, there were something like 17 Victorians at the Summer School this year across all three sciences, and to my knowledge, 5 of us scored raws of 50s in year 11. So far, the biology and physics teams have been announced, and out of the 4 Victorians chosen, 3 had 50s in year 11. One had quite a bit lower. Even though I 50ed physics in year 11, I don't think it helped me for olympiad - it's a completely different style (as in, it actually entails thought), and the knowledge that we covered in the 3/4 course was miniscule in comparison to what we did at olympiad camp.