You really do need to ask your Australian interlocutor, but he probably just meant mate (friend). The term is not specific to Australian English; I would have no idea what it might mean if uttered by "a person from the native tribe in Australia."
The Australian Oxford Dictionary (2nd edition) defines bo as
(colloquial) (as a form of address) pal; old chap.
and says the origin is US English.
Jonathon Green's Green's Dictionary of Slang (Oxford University Press)(over 100,000 entries) has three entries for bo and has more example uses for each than even the Oxford English Dictionary (OED). One meaning is that it is short for bohemian and I doubt that is the one you are asking about.
The most common meaning is
1 (also [spelled] beau, boh) a fellow, a man, a friend, often as a form of address, e.g. Hey, bo.
It gives usages ranging from 1825 to 1988, so according to Green, at least, the bo of today (found for example in the Urban Dictionary) is the same bo of almost 200 years ago. So that would include the usage by P.G. Wodehouse in V.V.'s answer.
Three examples from Green:
1867 W.H. Smyth Sailor's Word-Book (1991) 111: Bo. Abbreviation of boy. A familiar epithet for a comrade, derived probably from the negro.
1957 Kerouac On The Road (1972) 64: Say, bo, what was all the noise around here last night?
1978 D. DeLillo Running Dog (1992) 185: Hey, bo, come on down.
The above usage is short for boy and sometimes written bo'.
In Australian English and US English, bo also means
2 (Aus./US) (also [spelled] bow) a vagrant, a tramp.
Green considers this an abbreviation of hobo (so it is sometimes written 'bo), which Green defines as a tramp, a vagrant, an itinerant worker, often using the US rail system as a means of free transport. Obviously hobos in Australia wouldn't be using the US rail system.
I love this example (also found in the OED) which seems to offer an explanation of the term:
1893 Chicago Record 14 July 11/3: An' den w'en ye meets one uv yer own kind ye feels like old pals, 'cause he calls ye ‘Ho’ an' ye calls him ‘Bo’. See?
An example from 100 years later
1991 O.D. Brooks Legs 2: A bum that won't rustle food for himself […] is scorned by the bos that hustle for themselves.
Yet another definition in Green is
4 (US) a tramp's young homosexual companion; thus a young, effeminate male homosexual.
So, you might want to ask your friend whether he was calling you "mate" ("friend"), "bum" ("hobo"), "bohemian", or "queer" (homosexual).
You can see all these uses online in an abridgement of the above work, Cassell's Dictionary of Slang, page 143, halfway down the left column.