If it's a refundable fare the right thing to do is to call the company and tell them to refund the ticket since you won't be travelling. If there is an audit in the future they may come after you for the cost if you do fly. Probably not very likely, but possible. A fully flex fare to Australia in J is going to be expensive, even if it is a corporate discount fare.

If it is not refundable, or only against heavy fees, I would call the company and work out a deal with them, perhaps even offering to pay whatever minimal amount would have been reimbursed. Your argument is of course that the ticket has been paid for, the company is no worse off, and you could use the trip to strengthen business relationships or whatever. In any case, you come out as the honest person who did the right thing.