With 4 climbs over 200km and ~3500m ascent...is it the toughest day
ride for road cyclists in Australia?

Dunno, i did it in 06 and found that it was the heat (and the f*$king
flies!) that were the hardest to contend with..
the ride/climbs themselves are really not that bad, you go at your own
pace and theres plenty of flat/downhill to rest.

mostly i rekon its other factors which can make a ride tough or
easy..the 03? ATB was tough as it was 23deg and fine at the start, and
about 9 deg and raining by the time we got to the ferry crossing, the
following year was much easier