It is worth noting, vehicle claims alone were in excess of 400 million from this storm (which is more than the total bill, CPI adjusted, for previous such disastrous storms eg Western Sydney 1990, Brisbane 1985). Now, in comparison to the 1990 storm - the hailstones were undoubtedly larger in 1999 - evidence for 12cm hailstones as opposed to 8cm (1990). Could that be a reason why the media did not cover such internal damage to the same extent? There were less busted ceilings!
So, while I'd agree that the partial overlap in the track of these two supercells worsened the situation in the coastal suburbs (mostly water damage) and hence would increase insurance payouts I think the sheer size of the hail with the first supercell and also it's track were particularly significant in the 1.7billion figure. If the second supercell did not come through I dont think there would be a massive reduction in this. Also there was further rain after the event and many homes remained unprotected for sometime due to the building boom -- all of this weighs in. 10-12cm hail was the culprit.