LaTeX: [author year] numbered references using natbib

authoryear.bib

@book{Jacky:1996,
 author = {Jacky, Jonathan},
 title = {The Way of Z: Practical Programming with Formal Methods},
 year = {1996},
 publisher = {Cambridge University Press},
 address = {New York, NY, USA},
}

authoryear.tex

\documentclass[12pt]{article}
\usepackage{natbib}

\bibpunct{[}{]}{,}{a}{}{;}
\renewcommand{\bibnumfmt}[1]{[\citeauthor \citeyear]}

\begin{document}

\citep{Jacky:1996}

\newpage

\bibliographystyle{plainnat}
\bibliography{authoryear}

\end{document}