Make the local council build chicanes like that at each end of the town. Analyse that picture carefully, as both making them effective and not impeding truck traffic requires a specific shape and design. The warning signs should flash at night, as it may be easy to misjudge in darkness.
Google Street View,
52.077° N, 19.665° E (
OpenStreetMap)
Another option is building traffic lights, that turn red if they detect a speeding car approaching. Drivers routinely going that route learn quickly, that going over the limit will be less pleasant. Unfortunately only them: it doesn’t work for random drivers, which pass the location only once.
A third option is a speed display, which also displays a warning upon detecting speeding cars. This relies on drivers being reminded of the issue, which works quite well on many. Seems to be least effective, but also least intrusive and cheapest to deploy.
As for building anything yourself: I will join the choir of voices above. It’s likely to put you in legal trouble. Putting aside placing any device in road area without a permission — I don’t know NZ regulations on that — you expose yourself to liability from civil court cases, when something happens becuse of that flash.