I agree with Sir Humphrey when he writes that both OCs should claim against their insurer (who presumably will hound the car driver’s insurer).
That said, the following should help you (based on a detailed discussion I had a couple of years ago with a very capable Chairwoman of a strata scheme):
(a) The law that governs this is the Dividing Fences Act
(b) Step 1 (after neighbourly discussions get you nowhere) involves serving a FENCING NOTICE on your neighbour
Go to
https://www.lawaccess.nsw.gov.au
In the SEARCH BOX type
Fencing notice step by step guide
This will reveal in detail what you need to do.
(c) Step 2 (if/when Step 1 fails) involves a FENCING ORDER
At the above website, in the SEARCH BOX type
Applying for a fencing order
An application for a FENCING ORDER can be made at NCAT or the local court.
The SEARCH results will offer details about both options (via NCAT and local court)
The above websites are very informative. Good luck to you.
In the interest of self education, it cannot hurt you to ask your insurer if, assuming you make a claim (regardless of what your neighbour does), will you (our insurer) pursue the car driver’s insurer for this loss?
* Note I tried to include the full, direct, URLs for both fencing notice and order, but I was unsuccessful in
submitting it online.