Gassy odours from Mt Tongariro since it erupted on Monday night have been detected as far south as Wellington, as northerly winds blow the gas south.
On Wednesday morning, residents throughout the lower North Island have contacted councils to notify them that they have smelt the odour.
The Wellington City Council says it has taken at least two dozen calls from residents thinking the smell might be sewage in the harbour, or blocked pipes.
Residents have been told to call back on Thursday if the smell remains - in case it is a local issue.
The Manawatu/Whanganui Regional Council also says it has received reports from residents, especially in southern areas.
However, air-quality monitoring sites in Taumarunui and Taihape show no elevated presence of fine air particles.
GNS Science says it has heard from airline pilots who have noticed the smell at altitude and from several members of the public reporting a sulphurous smell.
Businesses hoping that's it
Businesses near Mt Tongariro itself are hoping the eruption was a one-off. They say the last time the volcano erupted, in 1897, it was active for many months.
The Department of Conservation has closed the popular Tongariro Alpine Crossing track, as well as four huts on the mountain. Falling rocks punched holes in Ketetahi Hut's roof, floor and bunks.
The chairperson of the National Park village business association, Murray Wilson, says closing the crossing will hurt dozens of families depending on it for their transport, guiding and support businesses.
He says, however, that it's hard to say what the full effect will be until DoC and GNS Science can reassess the hazards: "At this stage it's just day one of a multi-day or multi-week event."
Stewart Barclay of the guiding company Adrift says, "Yes, it will affect the business, but I'm hoping Mother Nature will stick to what she normally does, and that is, show her strength and then go and have a rest for another hundred years."
Mr Barclay says DoC has indicated the mountain is likely to be out for three to five days, and then he should be in more demand than usual.