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I'd say the sign was placed using a local map that followed what was until recently the most popular map datum in Australia, AUS66. Google Earth used WGS84 as its datum, and the difference between the two varies from around 120 metres to 400ish metres, depending where in the country you are.
Also, on Google earth the aerial photos are more often than not slightly out in their positioning on the grid. You can test this locally by setting your GPS to WGS84 (if it isn't already) and taking the position of a prominent object in your yard. It's best to average this position over half an hour or so if you unit is capable of it. Now go inside and compare the lat and long from your GPS position, to the lat and long of that item in your yard as seen on Google Earth. You'll most likely find the two don't line up.
Cheers,
Jason.
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