Air New Zealand says flights operating to some airports east of Mt Tongariro may be delayed because of the eruption.
And Airways, the company which operates air traffic control, said a no-fly zone of eight miles around the ash cloud is in place.
It said the cloud appears to be moving quite quickly.
Tongariro police have urged motorists to take extra care if they travelled to the area today to view the volcanic activity.
There were some road closures after the eruption but these were lifted fairly quickly and police were pleased the road network remained congestion-free for the rest of the afternoon and evening.
Area Commander Inspector Steve Bullock said they expected many people would want to visit.
"That is completely understandable and whilst we don't discourage people from now travelling to the area we are asking them to put road safety and their own safety before sight-seeing."
The main message is not to try and view the activity while you are driving; pull off the road. Also make use where possible of official car parks or areas well off the road to keep the roads clear from congestion and hazards.
While traffic flow was not normally heavy in the area it was fast-moving. People should be mindful that more traffic was expected in the coming days and take particular care when getting out of vehicles parked close to the road and when crossing the road.
The Alpine crossing remained closed but other walking tracks were open. Travellers to the area can get more information about what is accessible via the Ruapehu Area Department of Conservation Office.
Information and updates will also be posted on the Department of Conservation website - www.doc.govt.nz