When Did Australia declare war in WW1 World War One ?

Australia declared war in WW1 on Germany and Belligerents (Central Powers ) on the 5th August 1914.

Historical Dates from an Australian Perspective.
28th June 1914 - Assassination of Archduke Franz Ferdinand and wife, heir to Austro-Hungarian Empire.
28th July 1914 - Is accepted as the Start of World War One when Austro-Hungarian Empire declared war on Serbia.
31st July 1914 Friday -Prior to this date a Federal Election had been called in Australia. Thus the Prime Minister Joseph Cook and the Opposition Leader Andrew Fisher were electioneering when WW1 commenced. Two historic speeches were given on consecutive nights by the then Federal Opposition leader, Andrew Fisher (in Colac on Friday night the 31st July 1914),and followed the next night by Prime Minister Joseph Cook (in Horsham on the 1st August 1914). The two speeches declared Australia`s commitment to follow Britain into World War One, with Opposition Leader Fisher declaring "Australia will stand by the mother country to our last man and our last shilling" and Cook`s opposing speech on the Saturday night in Horsham with "If the old country is at war, so are we."
3rd August 1914 Monday - Germany declares war on France
3rd August 1914 Monday - Following Canada and New Zealand, PM Cook announces that Australia offers a 20000 man force to the Motherland.  
4th August 1914 Tuesday - Germany enters Belgium, Britain responds by declaring war on Germany.
5th August 1914 Wednesday - The then Governor General, Sir Ronald Munro Ferguson, received the cable that Britain had declared war on Germany at midday local time on 5 August. Prime Minister Cook then advised the people that Australia was at war with Germany. Australia did not have to 'declare war' as back then it had no constitutional right. Suffice to say, if 'Britain was at war', that meant Australia was at war.
The first hostile shot of the war (British and Dominions) occurred in Australia when the Victorian Fort Nepeanfire fired at the German merchant ship Pfalz trying to leave Port Phillip Bay.
10th August 1914 - AIF recruitment commences.
19th August 1914 - Australian Forces depart to capture German interests in New Guinea and Islands.
1st November 1914 - Convoy first AIF Leaves.

So It Begins, at the start of Great War Australia pledged 20000 men by the end of WW1, on the 11th November 1918, 330000 odd Australians had been sent to the front.
information sourced from Australian War Books