My opinion …. If it was a common property tree that caused the problem to the lot owner’s water pipe then the OC should pay for any unusual usage. The OC’s negligence is what has directly caused the damage that has created a large water bill.
If you use the ‘but for’ test the owner could claim that ‘but for’ the tree root from the common property tree his water bill would not be any different from usual.
The OC should pay the difference between what was the lot owner’s normal water usage and what was the unusually high amount caused by the damage from the OC’s tree.
You might want to take a look at the Trees (Disputes Between Neighbours) Act 2006 (Trees Act).