When playing both the Pacific and European theaters - you cross over when you move across the "3" line on the European movement map. It is 1 movement point and you are on the first sea zone on the Pacific map.
It is 3 movement points from the "Europe box" on the edge of the Pacific map to the West Coast of Australia (along the Soutern edge) and 3 MPs from the Europe box to Bombay India. The distance between Australia and Ceylon is also 3 (and to India proper, 4) - but to simplify movement when playing the European theater, we made it a consistent 3 between Australia and India.
Movement works out pretty much the same when playing both theaters or just one when trying to get units from one theater to the other.