Using natbib with agsm
With natbib
many aspects can be easily configured: you need to set citesep
(separation between two citations in the same place) and aysep
(separation between author and year in a citation):
\documentclass{article}
\usepackage{natbib}
\setcitestyle{citesep={;}, aysep={,}}
\begin{document}
\citep{Goossens:1994:LCb}
\citep{Knuth:1986:MB,Lamport:1994:LDP}
\bibliographystyle{agsm}
\bibliography{testbib}
\end{document}
(I used one of N. Beebe's databases.)
I think the following will do what you want:
\usepackage[comma,colon]{natbib}
based on this reference sheet