Auckland's bright lights are likely hurting local ecosystems, while partly masking the lunar cycle and ruining residents' view of the Milky Way.
That's according to researchers who measured night sky brightness over more than 100 Auckland sites across several years - and found industrial and commercial areas mostly to blame for harmful levels of glare.
While it might sound an unlikely type of pollution, what's called artificial light at night - or ALAN - can have large and widespread impacts on a multitude of species that depend on natural dark.
Globally, the problem has been shown to change the timing of dawn song in birds, stymie the nocturnal movement of small mammals and fatally attract phototaxic insect species to light.
"We often don't consider how much we have changed the night light-scape," said the study's lead author, Dr Ellery McNaughton, of the University of Auckland.
"A lot of that is familiarity - when you live in an urban area it becomes easy to forget that the sky you see at night is washed out and polluted, because we're not used to being able to see the stars and the Milky Way in all their glory.
"But the use of artificial light at night drastically changes the natural light cycles, and you can't change those without causing a raft of ecological impacts."
In the study, published in the journal Urban Ecosystems, 115 separate sites were measured for brightness for three years, while six residential sites were monitored continuously for 18 months.
The annual measurements were used to create maps of predicted levels of night sky brightness across Auckland.
These maps were then used to investigate the predicted extent of sky glow experienced by significant ecological areas, outstanding natural features and outstanding natural landscapes in Auckland - all areas specially classified for native biodiversity and other values.
In an earlier study last year, the researchers deployed audio recorders to capture birdsong from species like morepork/ruru at night and tui at dawn, along with devices to monitor pest mammals numbers and invertebrates such as flies, moths and beetles.
During the study period, streetlights were retrofitted from high-pressure sodium to LED as part of city energy-saving initiatives.
While that was found to have a limited impact on the wildlife studied - the dimmed, lower intensity mitigating the effect of LED's blue-rich light - the study authors said it still represented a missed opportunity to install ecologically-friendly lighting.
But the latest study revealed the harmful levels of light that species were being exposed to from artificial sky glow coming from various types of lighting.
In some years, more than 95 per cent of the areas the study team monitored were predicted to be experiencing night skies brighter than natural levels - and even up to 10 times greater at sites around the city centre and industrial areas.
At virtually all of the residential permanent monitoring spots, they found levels exceeded the threshold for "full human night vision and visibility" of the Milky Way.
While the artificial glare wasn't enough to fully mask the lunar cycle, the study still found the natural brightness of the moon was still being obscured.
That was a particular concern, as animals used the lunar cycle for basic processes and functions like reproduction and navigation.
Given all of these effects combined, the researchers found species on land - as well as some of those in oceans - were "likely to be adversely affected".
"To protect areas of ecological and natural importance, we should try to reduce light pollution wherever possible," McNaughton said.
"The good news is, unlike other forms of pollution, light pollution can be mitigated immediately as it doesn't hang around and there's no lag phase - reducing light pollution can be just as simple as flicking a switch."
She and her colleagues' work had shown that even making small changes to streetlights - like shielding, pointing them downwards, or dimming to 50 per cent in the quiet hours in the night - could all lessen impacts on wildlife.
"It's where lights are unshielded and brighter than they need to be that we have issues," University of Auckland ecologist Associate Professor Margaret Stanley added.
"Given we've documented these high levels of light pollution in Auckland, we should take some action to reduce the levels by modifying current lighting and thinking about how we use technology in our new industrial developments.
"We can also check the security lights we might have in the driveways of our own homes."