As far as I can tell, the lot owner is responsible for the damage to the unit where the leak occurred – which they can claim against their own insurance. And the Owners Corp is liable for the cost of repairs to common property, which they can claim against strata insurance. If there is an excess on strata insurance, the OC can claim charge that to the lot owner who can claim it through insurance or directly from the tenant.
But there is another point here – who is really responsible? If the plumber told the tenants to turn their taps on fully but didn’t tell them to turn them off, surely the plumber is partly responsible, at least.