Together with dinkum, cobber (a companion or friend), sheila (a girl or young woman), wowser (a puritanical or censorious person), and dunny (an outdoor toilet), bonzer (something excellent or first-rate) is a characteristic Australianism, typical of the lively and expressive slang of that country. Like the others, it is dated — only dunny and dinkum are now much heard in everyday speech.
However, bonzer had a good run from early last century (it was first recorded in a Sydney newspaper in April 1904) up to about the end of the Second World War, when it gradually began to fall from favour.
Bonzer was a general term of approval, so that if the weather was fine, for example, you might say “Bonzer sunny day!” This early example is in a verse from Songs of a Sentimental Bloke by C J Dennis, published in 1915:
This ev’nin’ I was sittin’ wiv Doreen,
Peaceful an’ ’appy wiv the day’s work done,
Watchin’, be’ind the orchard’s bonzer green,
The flamin’ wonder of the settin’ sun.
Where bonzer comes from is open to debate (one story, known to be untrue, says it comes from two Chinese words meaning “good gold”, a similar tale to one told about dinkum) but early examples suggest it may be from French bon, good, influenced by bonanza. The latter is Spanish, meaning fair weather or prosperity; as it was first used in US English in the 1840s for a successful gold mine, this is intriguingly parallel to the Chinese interpretation. If the mixed French/Spanish/American origin is correct, bonzer flowed from a linguistic melting-pot, appropriately for the country.