  An HI mass function (HIMF) was derived for 751 galaxies selected from the
deep Parkes HI survey across the Zone of Avoidance (HIZOA). HIZOA contains both
the Great Attractor Wall and the Local Void, two of the most extreme
environments in the local Universe, making the sample eminently suitable to
explore the overall HIMF as well as its dependence on local environment. To
avoid any selection bias because of the different distances of these
large-scale structures, we first used the two-dimensional stepwise
maximum-likelihood method for the definition of an average HIMF. The resulting
parameters of a Schechter-type HIMF for the whole sample are $\alpha =
-1.33\pm0.05$, $\log(M_{\rm HI}^*/M_{\odot})=9.93\pm0.04$, and $\phi^* =
(3.9\pm0.6)\times 10^{-3}$ Mpc$^{-3}$. We then used the $k$-th
nearest-neighbour method to subdivide the sample into four environments of
decreasing local density and derived the Schechter parameters for each
subsample. A strong trend is observed, for the slope $\alpha$ of the low-mass
end of the HIMF. The slope changes from being nearly flat, i.e. $\alpha =
-0.99\pm0.19$ for galaxies residing in the densest bin, to the steep value of
$\alpha = -1.31\pm0.10$ in the lowest density bin. The characteristic mass,
however, does not show a clear trend between the highest and lowest density
bins. We find similar trends in the low-mass slope when we compare the results
for a region dominated by the Great Attractor, and the Local Void, which are
found to be over-, respectively underdense by 1.35 and 0.59 compared to the
whole sample.
