Green’s Dictionary of Slang

Mallee bull n.

used as a notional measure of health and/or strength.

In phrases

fit as a mallee bull (adj.) (also strong as a mallee bull)

(Aus.) extremely powerful.

[Aus]Australasian (Melbourne) 21 June 6/1: I was as lissom as a quince bough and as strong as a mallee bull in them days.
[Aus]Canberra Times (ACT) 18 Oct. 2/3: ‘I have a check-up twice a year and my doctor has told me I'm fit as a mallee bull’.
[Aus]Canberra Times (ACT) 4 Jan. 16/1: ‘Sir has trained down to almost peak fitness. [...] generally Sir is as fit as a mallee bull’.
[Aus]J. Byrell Lairs, Urgers & Coat-Tuggers 43: Archer was not only as strong as a mallee bull but as fast as a scared possum.
[Aus]P. Doyle (con. late 1950s) Amaze Your Friends (2019) 92: ‘How are you, Joe?’ ‘Fit as a Mallee’.
[Aus]G. Gilmore Base Nature [ebook] ‘It’s going to take two of us to overpower him. He looks fat but he’s like a Mallee bull’.