I'd avoid BGC

I find this a bit odd, I would've thought the footpath/kerb bond was designed to cover this
Our builder said that when we paid these costs in our provisional site works…

I picture it the same as them hitting a power pole outside your block.
You wouldn't have to front up money for that.
They broke it.

How is this your issue? If one of the builders trades did it then they need to fix it. If the builder suspects sub standard kerb they need to fix…

I wouldn't pay myself - like someone said, they did the damage not you and I would expect them to repair it. Don't many builders pay like a footpath kerb…