Australia's raging bushfires earlier this month saw an "unprecedented" spike in carbon monoxide levels monitored in New Zealand, NIWA says.
Data from the isolated Lauder weather station, in South Otago, measured a drastic increase in carbon monoxide in the air.
In early December, it showed a jump to 150 parts per billion (ppb). But this month, it shot up even higher to 500ppb.
NIWA called it an "unprecedented" measurement.
"We see some elevated carbon monoxide from [local fires in South Otago], but nowhere near this level," Lauder-based atmospheric scientist Dr Ben Liley told 1 NEWS.
He says it's unusual for the smoke plumes from the Australian fires to have reached ground level in New Zealand.
Earlier this month, skies in parts of New Zealand turned yellow and orange as smoke drifted across from Australia.
That was due to particles in the atmosphere, at a higher altitude, Dr Liley says, while the carbon monoxide reaching the ground is a colourless gas.
However while these levels of 500ppb seem stark, it's not nearly enough to cause harm to people at ground level.
Health issues from carbon monoxide can occur between 10 to 30 parts per million, Dr Liley says.
In comparison, these measurements are around 0.5 parts per million - "well below the level of toxicity".
"It's not a threat to us, but it's a clear sign the smoke is both coming across here and is mixing down to the surface, not just wafting over at high altitude," Dr Liley says.
Local fires can also increase the carbon monoxide registered at Lauder, but Dr Liley says they're usually brief spikes instead of prolonged periods.
"The interesting thing about this is that it has persisted for the whole day," he says.
The levels of carbon monoxide have since returned back to normal, at around 60ppb.