On a CSG linux machine the LaTeΧ class file is: /usr/local/share/texmf/tex/latex/unsw/unswletter.cls
For information on the rest of the layout of your CSE letter use the command: man letterhead
or read the comments in the unswletter.cls file.
\documentclass[colourcrest,cse]{unswletter}
\name{Joe Bloggs}
\email{j.bloggs@unsw.edu.au}
\begin{document}
\begin{letter}{(Where this letter \\ is coming from.)}
\opening{Dear Recipient,}
Some text.
Some more text.
\closing{Regards,}
\ps \ldots Some postscript comments.
\end{letter}
\end{document}
For Win or Macintosh you can download the template from our web site at: http://mirror.cse.unsw.edu.au/pub/letterhead/ (Not currently updated to latest format.)