Sure, Kirsty, I can help you out with this one.
It's a very nice aneroid barometer mounted in wood with a decoratively carved rope edge that dates it to about 1890 - 1914, when this was a fashionable way of wall mounting a barometer.
It also has a thermometer incorporated into the dial, an unusual feature of one of these, indicating that yours was once a working ship's barometer, located on the bridge where the temperature indicated was indeed the true outside temperature, rather than merely an indoor room measurement as it would be now.
The calibrations around the outside chapter ring of the dial, namely: Stormy, Rain, Change, Fair, Very Dry are what one would see on a standard barometer (NB. these are indicators of predicted weather, rather than what is currently happening).
Around the inside are what's called "Ellery's Indicators" that add extra calibrations specifically designed for Antipodean weather and barometric pressure (which has a wider range than in the North Atlantic, say, for which most barometers are calibrated). In other words this barometer is calibrated for specific use in Australia and New Zealand. That, and the thermometer combination, make this a very rare instrument.
If you were to see it for sale in an antique store it would have a full retail price tag of AUD$1600.
Expect to get 30% - 40% of this if you were to sell at a good antique auction house.
I do hope this helps!