I'm not sure of the extent of the Victorian government's involvement in this debacle, but it is clear they had expressly gave him their assurance (multiple sources mention both Tennis Australia and the Victorian government gave him their approval, even if the visa was ultimately wrong) and were aware of the circumstances (would be surprising if they weren't attentive to the world's #1 tennis player coming to one of the world's most prestigious tennis tournaments).

I think from his side it's a weak argument to follow but maybe it makes sense in a common law context - whatever instance has given him a go-ahead, he's going to use that and say he had some kind of assurance. It's just not from the right department or level at all, but he can try. He needs a pseudo good faith argument for "I thought I could go", and that's what he's trying to do - that's his foot in the door, then the courts decide.