It is black with yellow dots down each side of its body.

Any ideas ?

I have just been flicking them off at this stage so they are not a serious problem, I am just curious.

http://www.ento.csiro.au/anic/resources.html - the csiro entomology web page

http://linus.socs.uts.edu.au/~don/larvae/butter.html - australian buttterfly larvae

hope these help
http://linus.socs.uts.edu.au/~don/larvae/papi/papilionidae.html - some more of the butterfly link