They replied and are adamant the bag is authentic and they also said they would pay for cost of repair and they will also pay to get it authenticated by Givenchy in Melbourne.
I would doubt very much that Givenchy in Melbourne would authenticate your item, even if paid. A lot of stores don’t do this. Please see what Givenchy has adviced above.
Email Givenchy yourself and go to ACCC with Givenchys confirmation that they do not distribute to this store.