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Transportation to Australia (as the criminal sentence was labelled) was not necessarily for life (although, the cost of returning to Britain essentially made it so). Many crimes that afforded a sentence of Transportation would seem to be quite petty by today's standards (stealing a loaf of bread, for example, or even just being in debt) - and the court/magistrate would state a period of service after which the transported convict would become a free person again.
So, your bread-stealer might get 5 or 7 years transportation, for example, and as long as they worked in the colony and kept out of any other trouble, they would become a free man at the end of the period. Quite a few ex-convicts eventually become important businessmen and leaders in government - some return to Britain, and others moved on elsewhere.
The wiki article makes it sound like there was simply a mass exodus in one fleet from Australia to San Francisco - but more likely, this happened over a period of time as ships with trade goods arrived from Australia with freed transportees looking to get away from the colony. As for why they would want to move to San Francisco - the 1840s was the gold rush period.
Upvote:1
Because they were "former" members of the penal colony, or "ex-convicts," with "tickets of leave." That is, they served a term in the their Australian "prison," and once their terms were expired, they were free to go where they pleased, provided they could earn the money.
Some elected to stay in Australia. But others elected to go to the nearest free, English speaking place that was not of the British Empire. That would have been San Francisco.
Put another way, the "weren't moved" (by the authorities). They "moved" themselves.