I fly SYD<->YVR at least once a year (skiing at Whistler of course) on award tickets using another *A airline's points (i.e. not AC) - and in the past couple of years here's my observations on getting business class awards trans-Pacific:-

UA is the easiest except for last (2012 ski season) year for some reason - availability for 2013 looks good if you book 330 days out.

AC is next best - and I have definitively snagged seats at 330 days (Mon-Wed but sometimes even on a Fri or Sat) OR at about -3 months from date of travel so sometimes I book UA (or OZ) as a fallback option then pay the change fee to get AC seats if and when they become available closer in.

The AC/OZ option via ICN is very available (except for Jan and Aug when OZ have blackout dates - you need check the Asiana web site to get exact dates) - ie YVR-ICN on AC then ICN-SYD on OZ but this is a much longer trip and requires an overnight in ICN when travelling SYD-ICN. You can also route on OZ or UA via LAX or SFO which may or may not be convenient depending on your final destination.

Now onto NZ - they fly daily AKL-LAX and SFO but only 3/4 times per week to YVR - all of these routes are very hard if not impossible to get at 330 days but at -4 months you do sometimes see availability. Unless you want to stop in AKL it's not all that convenient a routing IMO and most flights AKL-SYD are Y only flights and even though NZ have 10+ flights per day across the Tasman award seats during Jan (school holidays here) are pretty much impossible to get at any time.

Now as for the difference between AC/UA/NZ/OZ in business - they are all good IMO so I choose whichever gives me the most convenient routing (assuming awards are available).