Yes it certainly is, but a much-modified one.
It looks like it started out as a 3 1/2 HP (500cc) water-cooled model of the type current between late 1910 and 1912. The registration number was issued much later, so at a guess the photo might have been taken after the war.
Modifications are the valve enclosure, footboards instead of the usual pedals, extended air intake on the carburettor and the dropped handlebars. Magic!
The engine is a Precision (made by F.E. Baker Ltd. in Birmingham) and the frame is made from Chater Lea fittings. On this model the radiator occupies the front section between the tank rails, and is separate from the fuel and oil tank behind it.
Lewis built probably 2000 machines in Adelaide, South Australia between 1899 and the mid 1920s. For more information check out The Lewis Project website:
http://users.senet.com.au/~mitchell/lewis/If anyone has Lewis photos or information, I'd love to hear from them.
Cheers
Leon