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Lachlan Macquarie was an early (1810-1821) governor of the New South Wales colony, and his name was given to the Lachlan River, which subsequently gave its name to a number of administrative areas (Lachlan Shire, a Lachlan electorate, and the like). Media magnate Rupert Murdoch named his son Lachlan in 1971, which may have helped raise the profile of the name in recent times.

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I ran a historical search on it on the NSW births website, and it was used semi-regularly (289 times but some are multiples, probably more like 250 times) between 1788 and 1906.

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But really, all I can tell you is that it appeared in the Top 100 in 1979 and hasn't left it - it's been steadily increasing in popularity ever since. Most of the currently famous Lachlans were named during this time (they're celebs around my age) and any characters named Lachlan were given the name because of the trend. So I really don't know what caused it!