A polite note asking on what grounds they are charging you for repairs to resolve their problem, would be in order
However, if the piping on your common property was causing a problem for them, at least in part, you are lialble for some of the cost but you are entitled to know why your OC wasn’t informed or consulted.
But if the work done on your section of the pipe was purely so they could get access to the problem in theirs, I wouldn’t be rushing to pay the bill.
A sit-down and a chat could resolve this but if they are just trying to pass part of the expense for their problem on to you, I would refuse to pay until they come up with something more convincing than a demand for money.
By the way, NCAT has the ability to resolve disputes between adjoining strata so you could always tell them to go there if they aren’t happy.