The Snider was my thought too, but am still uncertain due to the shape of the final few inches of the fore-end. The picture seems to suggest a long tapering single section towards the muzzle, where as the Snider has a distinctive 2 stage step section at this point.
Could be more about the shadows/angle of the picture but the similarity to the Sharps is pretty strong......
...but then, if it is a Sharps were they exported/available on the Gold Coast?