locates not proper and hit gas line

Hello all...

I am a home builder and was doing digging with excavator...I had locates done and where I was digging there were no markings..

Anyway..we hit a gas main and the utilities company are saying I am responsible. .

Do I have any ground to stand on?
There are saying I'm responsible for $16000 ..

I am in Toronto Canada if that makes any difference. .

Thanks

locates not proper and hit gas line