Why did the gold rush begin?

Source: http://www.australianminesatlas.gov.au/education/down_under/gold/australia.html
In 1823 James McBrien found traces of gold near Bathurst, NSW. However, early discoveries of gold in Australia were hushed up by the authorities for fear that all the convicts, soldiers and public servants would stop work to hunt for their fortune.

In 1841, the Rev. WB Clarke found a gold nugget near Cox's River in the Blue Mountains, NSW. When he showed the gold to Governor Gipps, the Governor said, 'Put it away, Mr Clarke or we shall all have our throats cut!'
It wasn't until ten years later, in 1851, that Edward Hargraves, (who had just returned from the gold fields in California) and his colleagues found gold near Bathurst. This time the find was publicised and within a month a thousand men were looking for gold. The area was called Ophir, after the biblical story about King Solomon's gold city.

Source: http://australia.gov.au/about-australia/australian-story/austn-gold-rush

In 1851, Edward Hargraves discovered a 'grain of gold' in a waterhole near Bathurst. The discovery marked the beginning of the Australian gold rushes and a radical change in the economic and social fabric of the nation.

Source: http://www.nma.gov.au/interactives/tlf/gold_rush_5-6/index.html
Gold rush interactive
Gold rush interactive

Source: http://www.sbs.com.au/gold/story.php?storyid=30
"A complete mental madness appears to have seized almost every member of the community, and as a natural consequence, there has been a universal rush to the diggings." - The Bathurst Times

Source: http://www.goldrush.com.au/index.php/gold-attracts-immigrants-worldwide
The discovery of extensive gold deposits in New South Wales and Victoria in 1851 and 1852 opened the second major phase of European migration to Australia. As news of the discoveries spread round the world, hundreds of thousands of people flocked to the Australian colonies in the hope of becoming rich.