But a list of notable decisions has finally emerged. It's far from comprehensive, mostly about the general approach and public interest, and may not do anything like justice to the topic so comments, suggestions, additions and even quibbles are welcome.
Thanks to Professor Moira Paterson of Monash University, Carolyn Adams from Macquarie University, Michael McKinnon of the Seven Network, and Sven Bluemmel and Rachel Krute from the WA Information Commissioner's Office for inputs, but they bear no responsibility for what follows.
The cases aren't "leading" necessarily, but in my view had an impact for good or ill (too few of the former, too many of the latter) in moving things along over the last thirty years toward or away from that bright shining object of extending the right of access to government information.
As Carolyn Adams commented, things have moved a long way since R v Southwold Corp; Ex p Wrightson (1907) 97 LT 431 where the court in relation to the common law observed that ‘mere curiosity and desire to see and inspect documents’ was not sufficient to entitle a person to view public records.
Of course perhaps the best known case, the High Court decision in McKinnon v Secretary, Department of the Treasury (2006) 228 CLR 423, is hard to categorise. On the one hand it was disappointing that the majority were prepared to stoke some hoary old public interest against disclosure chesnuts, and that French CJ and Kirby J in the minority didn't grab the opportunity to write pages and pages dispelling such notions and lay a foundation stone for contemporary public interest considerations. On the other it proved to be camel back-breaking in effect, prompting lobbying efforts by media organisations (and a few others) to push for change that were picked up federally by Labor in opposition, and subsequently led to Federal and some state reforms. Notably the case proved to be the deathknell of conclusive ministerial certificates - except in Victoria and Western Australia where they remain in law but apparently are rarely used.
The Federal Court in Harris v Australian Broadcasting Corporation  50 ALR 551 got things off to a promising start by emphasising that the public interest test required both sides-for and against disclosure- not just a focus solely on the reasons not to disclose. There must be
"a careful balancing of the public interest in citizens being informed of the processes of government against the public interest in the proper functioning of government."[ 561]
On the other hand the AAT in Waterford and Department of Treasury (No 2)  5 ALD 588 took a broad brush to the meaning of "deliberative processes" and decided the term covered all the thinking processes of an agency involved in its functions, pushing the envelope some thought.
Then the Federal Court decisions News Ltd v NCSC (1984) 1 FCR 64 and Searle Australia Pty Ltd v Public Interest Advocacy Centre and Department of Community Services and Health (1992) 36 FCR 111 kicked off and cemented in place the unfortunate "no leaning in favour of disclosure approach" to the objects clause which paved the way for broad interpretation of exemption provisions thereafter. Now less of an issue in 2013 perhaps as a result of the 2010 reforms, but "no leaning" is still lurking when it comes to non-exemption provisions. (Searle deserves other 'notable' mentions: because of the acceptance of a broad interpretation of the expression trade secrets, significant as a trade secret is exempt per se irrespective of demonstration of commercial harm; and because of rejection of possibility of considering identity of applicant in assessing application of exemption.)
Oh what might have been if instead, this emphasis on the objects had been plucked out of the High Court's unanimous judgment in Victorian Public Service Board v Wright [1986 CLR 145], a case involving the Victorian FOI act, and had come to guide decision makers and the courts :
In addition( to the objects section) s.16(1) requires Ministers and agencies to administer the Act with a view to making the maximum amount of government information promptly and inexpensively available to the public. In the light of these sections it is proper to give to the relevant provisions of the Act a construction which would further, rather than hinder, free access to information.That approach was reflected in some state cases including in Victoria in Accident Compensation Commission v Croom  2 VR 322 and in Sobh v Police Force of Victoria  VR 41 and in NSW in Commissioner of Police v District Court of NSW (Perrin) (1993) 31 NSWLR 606.
However when it came to the public interest in the deliberative process, administrators, tribunals and courts particularly at the Commonwealth level embraced the wisdom in Re Howard and Treasurer of the Commonwealth (1985) 7 ALD 626 and made good use for years of factors cited by Justice Davies as constituting public interest considerations against disclosure, factors that were inconsistent with the objects of the Act. Disclosure would cause "confusion and unnecessary debate" is an all time favourite. Most including that one were legislated out of existence in the Commonwealth 2009-2010 reforms but 'frank and fearless' have been given renewed life in the Attorney General's Terms of Reference for the Hawke review of the act.)
On scepticism about frank and fearless, the duties of public servants and analysis of every minute detail of these issues, plaudits to Deputy President Forgie's monumental decision in McKinnon and Prime Minister and Cabinet  AATA 1969, which I'm sure Dr Hawke will find interesting. (I haven't checked but from some old notes, arguments for frank and his friend were rejected in other earlier cases including Sunderland, Witherford, Fewster, Murtagh, and Chandra.)
More positive approaches too, to the appropriate standard of transparency and accountability we should expect were evident in Queensland Commissioner Albietz decision in Re Ecclestone and Department of Family Services and Aboriginal and Islander Affairs (1993) 1 QAR 60 and the Full Court of the Supreme Court of NSW decision in WorkCover Authority of New South Wales v the Law Society of New South Wales  NSWCA 84.
The Victorian decision DPP v Smith 1 VR 63 deserves a mention because of the succinct description, oft cited since, of the public interest as
“…a term embracing matters, among others, of standards of human conduct and of the functioning of government and government instrumentalities tacitly accepted and acknowledged to be for the good order of society and for the well-being of its members…events of interest to the public may or may not be ones which are for the benefit of the public.."The High Court in Osland v Secretary to the Department of Justice  HCA 24 illustrated the case for a public interest override for all exemption categories, powers only held currently by the Victorian Civil and Administrative Tribunal to a degree (s 50(4) of the Victorian Freedom of Information Act). The court upheld the decision of the President of the Victorian Civil and Administrative Tribunal Justice Morris in August 2005 which held that the differences between the advices from senior counsel to the Victorian Attorney General whether to grant a petition of mercy on behalf of Mrs Osland provided the "powerful reasons" for requiring access to the documents in the public interest. In the circumstances of this case, the Court decided public interest considerations overrode the valid claim of legal professional privilege, and granted access to the 300 pages of advice received by the Attorney General from all sources. There are absolute exemption categories in Federal and all state laws. You can only wonder how many important public interests in disclosure have been ignored, lawfully, as a result. Hopefully governments at some stage will see the light.