According to trusty BRW; in Australia, according to Budgeted Fees for 04-05, PKF was on 108.5m with 110 partners (Rev per partner of 986,364) and BDO was 102.0m with 95 partners (Rev per partner of 1,073,684); so that leaves PKF ahead slightly. Not taking into account Investor Group as turtleface pointed out, that would leave PKF in 5th and BDO 6th.
Source: BRW, p70, July 28-Aug 3, 2005
However, also according to BRW, worldwide, BDO is ranked 5th with revenue of 3,017m (US) and having 621 offices in 105 coutnries, and Grant Thornton about 33% behind on 2,092m (US) in 6th with 504 offices in 109 countries.
Source: BRW, p71, Aug 18-Aug 24, 2005