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Unfortunately not. There are very few international 'tag' flights' in Australia, even fewer (if any at all) that have First. I believe United has one between Melbourne and Sydney before going onto the US. I'm sure if that aircraft has an F cabin (I don't think it does) but even if it did it is not possible to sell a ticket just for MEL-SYD, in the same one a person can't fly that leg on its own in Economy.