The Transport Agency is trying to make it harder for people to throw objects off motorway overbridges onto the traffic below.
Police figures show 27 reported incidents of objects thrown from Auckland overbridges between July and September this year.
NZTA state highway manager for Auckland, Tommy Parker, says it is difficult to completely stamp out the practice, because if someone is determined to throw something off, then it is hard to stop them.
But bridges are now being built with high fences that curve inwards, he says.
The figures, obtained by Radio New Zealand, show that almost half of the reported incidents occurred on the southern motorway.
Only two people have been arrested in relation to the incidents.
Penalties too soft, says man whose son died
Wayne Currie, whose son Chris died in 2005 when a chunk of concrete was thrown off an overbridge and smashed through his windscreen, says penalties should be harsher, because those throwing debris have no fear of the consequences.
The vast majority of objects were thrown between one and six in the afternoon, with the peak between three and four o'clock. Mr Currie says that suggests that young people walking home from school are mainly to blame.
But the officer in charge of the Auckland Motorways Unit, Inspector Dave Walker, is not so sure about that; he says more people would have to be caught and their ages recorded before it could be confirmed.
Almost a quarter of the incidents occurred on Saturdays.
NZTA says that, after several incidents, two safety barriers were installed in 2007 and 2008 at Alfriston Road and Princes Street in South Auckland.