  Let $G = N \rtimes A$, where $N$ is a graded Lie group and $A = \mathbb{R}^+$
acts on $N$ via homogeneous dilations. The quasi-regular representation $\pi =
\mathrm{ind}_A^G (1)$ of $G$ can be realised to act on $L^2 (N)$. It is shown
that for a class of analysing vectors the associated wavelet transform defines
an isometry from $L^2 (N)$ into $L^2 (G)$ and that the integral kernel of the
corresponding orthogonal projector has polynomial off-diagonal decay. The
obtained reproducing formula is instrumental for proving decomposition theorems
for function spaces on nilpotent Lie groups.
