Encouraged by some offlist help, I have taken the time to scan the colour
view, which shows the date: 1860.
Auburn (SA): 'Australia's first horse tram'. Fri.30.5.69. (Roderick B
Smith)

They can't even be bluffing with Victor Harbor. If they are trying to use
the route on which the vehicle plied, they could have used 1854, or the 1864
extension to Victor Harbor.
When opened to Port Elliot, single-deck open vehicles were used.
'South Coast Limited' (Kim Bird, 1972) has lots of photos of vehicles in the
horse era.
They were to be single deck to 1879, when the first double-deck trams were
imported, but not of the style at Auburn. They were described as similar to
the ones in use in Adelaide.
Now what is needed is the date for the first horse tram in Adelaide, and
others will have it more readily than I do.

Roderick B Smith
Rail News Victoria Editor