The first author in academic writing to use author-date in-text citations with a matching alphabetically ordered reference list was a Harvard academic. Before that academic writing used numeric in-text citations, footnotes, and reference lists in order of first citation.
From this first example, multiple publishers and editors have adapted author-date citing and referencing styles that they have named "Harvard style". There is no one authoritative Harvard style.
This guide for in-text citations and reference lists is based on the following style manual:
Commonwealth of Australia 2002, Style Manual for authors, editors and printers, 6th edn, rev. by Snooks & Co, John Wiley & Sons, Milton, Australia.
This style is sometimes known as the Australian Harvard style, and has been used by a range of Australian publishers and editors.
As online resources have changed dramatically since the style manual was published, local variations in some of the online referencing examples have developed, so do not be alarmed if you see variations in examples from other sites.
Achnowledgement: Parts of this guide have been adapted from the Monash University Harvard Referencing Guide. More examples may be found in that guide.