I believe it was a private jet. The same one used by Taylor Swift recently. There's some details on this here - https://protos.com/crypto-community-wants-to-fund-julian-assanges-jet-expenses/

I found it really interesting that Assange was charged for the chartered jet by the Australian government in the first place. It does make sense though that Australia didn't want to burden their taxpayers which is a nice change for once..... 