The colloquial Australian-English phrase (as) game as Ned Kelly means very spirited or brave.

More generally, in Australian English, the name Ned Kelly is applied to a person of reckless courage or unscrupulous business dealings.

This refers to the Australian outlaw Ned Kelly (Edward Kelly – 1855-1880), who was the leader of a band of horse and cattle thieves and bank raiders operating in Victoria, and who was eventually hanged in Melbourne.

With reference to the courage that Ned Kelly showed on the scaffold, the phrase as brave as Ned Kelly occurs in the following from The Herald (Melbourne, Victoria) of Monday 15^th January 1894—Frances Knorr (1868-1894), an English migrant to Australia, had been found guilty of strangling an infant:

Chief-warder Long, who has seen about 25 executions, stated that he had never seen one which passed off more perfectly, in every technical detail. Of the demeanor of Mrs Knorr on the scaffold, this officer speaks in high terms of praise. “No mistake, she was plucky,” he remarked; “she was as brave as Ned Kelly.” Mr Long has a vivid recollection of the demeanor on the scaffold of the chief of the Kelly gang, and his comparison of Mrs Knorr’s death with that of the celebrated desperado conveys a good idea of the fortitude she displayed.

These are, in chronological order, the earliest occurrences of the phrase (as) game as Ned Kelly that I have found:

1-: From Working a Passage Overland. From Sydney to Melbourne. The Tale of a Tramp, by ‘J. C.’, published in the Evening News (Sydney, New South Wales) of Saturday 11^th May 1907:

Being unable to raise the fare to Melbourne, my mate and I decided to walk the 582½ miles.
We left Sydney on a very wet night recently […].
[…]
Next day found us in Picton […] Staying in Picton that night, we were telling a farmer we were bound for Melbourne. He laughed, and said we were as game as Ned Kelly. He told us if we liked to take a little risk we could jump on a goods train and get a lift; that at Hilltop there was a very steep grade for the trains, and we could easily hop on one going south with empty trucks.

2-: From the account of a Rugby-League game between Newcastle and Sydney, published in the Sydney Sportsman (Sydney, New South Wales) of Wednesday 22^nd September 1915:

The game itself was one of the most bloodthirsty affairs seen in Sydney for years, the main object of every player on either side seemed to be to get as far away as possible from the ball so that somebody else would have to pick it up and thus give the ones standing off a chance to gig at their wild attempts to snare the tricky inflated pigskin.
[…]
There was a cannibal in the front row, who would persist in biting everyone he came in contact with, Rattle Wright for preference.
The Newcastle boys reckon that Dumpy Downs, Froggy French, and Shiver Jones were as game as Ned Kelly to put up with the tearing about the three got from somebody’s number tens in every scrum.

Susan Wyndham evoked Ned Kelly’s cultural significance in Off with his mask, the review of True History of the Kelly Gang (Brisbane: University of Queensland Press, 2000), a novel by the Australian author Peter Carey (born 1943)—review published in The Sydney Morning Herald (Sydney, New South Wales) of Saturday 30^th September 2000:

Peel back Ned Kelly’s iron mask ¹ and what do you find? Another mask, another layer to the legend. Kelly lives on, 120 years after his gang of bushrangers was routed and he was hanged, as Australia’s symbol of anti-authoritarianism, Irish-spirited republicanism, underdog bravery, troubled youth. He’s branded on our culture (try a Kelly Country soft drink – Larrikin Lime, or Reckless Raspberry) and our language (“game as Ned Kelly” is still a compliment). But the man remains elusive. We can never know the whole truth about him; we can only invent it.
That’s why Kelly obsessed Sidney Nolan ² for 40 years. Why there had to be dozens of Neds in Kelly’s Republic at the Sydney Opera House in 1997 ³, and why they had to be brought back for the Olympics opening ceremony ⁴. Why the story will be told again in a movie by historian Don Watson ⁵ and Wildside’s Michael Jenkins ⁶ next year. And why Peter Carey has written a novel with the self-mocking title True History of the Kelly Gang.
The vast documentary material is just enough to stir Australia’s writers and artists into endless speculation. For Peter Carey “the whole story is like a huge, dark plain in which, magically, certain moments are spotlit. Look at it from a distance and you see a lot of black with these pinpricks of light. The rest is unknown, unimagined.”
Pamphlets, songs and potboiler novels were being written even while the gang was roving north-eastern Victoria at the end of the 1870s. Australia’s first full-length film, made in 1906, was The Story of the Kelly Gang ⁷. (Four more films of dubious value followed, including Mick Jagger’s 1969 rock ’n’ roll Ned ⁸.) A verse play, Ned Kelly, by the New Zealand-born poet Douglas Stewart was broadcast on radio in 1942 ⁹. Albert Tucker ¹⁰ painted his monumental Kelly.
But the image that sits in the Australian subconscious comes from Sidney Nolan’s series of 27 canvases painted in a rush of energy and postwar anger between 1945 and 1947. Nolan put the empty mask firmly on Kelly’s shoulders. As Andrew Sayers, now director of the National Portrait Gallery, wrote in a 1994 catalogue for the Metropolitan Museum of Art in New York: “The slotted black box that Nolan made of Kelly’s helmeted head has become an instantly recognisable icon; Nolan’s Kelly has become Ned Kelly.”
For a long time Nolan seemed to stymie any need to re-examine Kelly. Only the 100^th anniversary of his death in 1980 prompted a new round. Histories were written, such as John McQuilton’s The Kelly Outbreak (Melbourne University Press, 1979) ¹¹; Graham Seal’s Ned Kelly in Popular Tradition (Hyland House, 1980) ¹²; Keith McMenomy’s Ned Kelly: The Authentic Illustrated Story (Currey O’Neil Ross, 1984); and, later, Ian Jones’s Ned Kelly: A Short Life (Lothian, 1995).
It is the fiction of the past 20 years, however, that tells us most about Kelly’s meaning for contemporary Australians, that brings us closest to the man. There have been three major novels: Sister Kate by Jean Bedford ¹³ (Penguin, 1982), Our Sunshine by Robert Drewe ¹⁴ (Pan Macmillan, 1991) and True History of the Kelly Gang (published next month by University of Queensland Press). For each of them it has been a seminal work. Together they have put flesh on the man, and filled the helmet.

¹ Ned Kelly and his gang forged their own body and head armour.
² Sidney Nolan (1917-1992) was an Australian artist whose Ned Kelly paintings became iconic.
³ Kelly’s Republic, written and directed by Nigel Jamieson, was staged at the 1997 Sydney Festival; Anthony Steel was the director of the Sydney Festivals from 1995 to 1997.
⁴ Nigel Jamieson was part of the team that created the opening ceremony of the 2000 Summer Olympics in Sydney.
⁵ Don Watson (born 1949) is an Australian author and screenwriter.
⁶ Wildside (1997-99) is an Australian television series created by the Australian film and television director Michael Jenkins (born 1946).
⁷ The Story of the Kelly Gang (1906) is an Australian film written and directed by Charles Tait (1868-1933).
⁸ Ned Kelly (1970) is a British-Australian film directed by the British film director Tony Richardson (1928-1991), and starring the British singer, songwriter and actor Mick Jagger (born 1943).
⁹ Ned Kelly (1942) is a radio play by the New Zealand-born Australian poet Douglas Stewart (1913-1985).
¹⁰ Albert Tucker (1914-1999) was an Australian artist.
¹¹ John McQuilton (born 1948) is Associate Professor and Principal Honorary Fellow in History at the University of Wollongong.
¹² Graham Seal is Professor of Folklore at Curtin University.
¹³ Jean Bedford (born 1946) is a British-born Australian author.
¹⁴ Robert Drewe (born 1943) is an Australian author.