At least 10 flights had to be cancelled and three delayed from Tauranga and Rotorua airports this morning after an eruption.
GNS Science is investigating. It said last week a new vent was detected in the volcano and there was an increase in ash.
Images showed the column of smoke emitted from Whakaari growing considerably in size in the two hours to 8am today.
Air New Zealand said volcanic ash had been affecting the flight path for both airports.
At 11.30am, flights had resumed and were operating as normal in and out of Tauranga, the airline said.
"We are continuing to monitor the situation to determine whether any further flights in or out of Rotorua will be impacted, and advise customers travelling today to check the Air NZ app or our website for the latest information on their flight.
"The decision to cancel or delay flights this morning was a proactive safety measure."
Air Chathams said it would monitoring conditions for flights to Whakatāne this afternoon.
Chief commercial officer Duane Emeny said one of its aircraft was able to make a return trip there this morning.
GNS volcanic duty officer Brad Scott said the volcano had been in a state of minor eruption for several weeks now - but it was not aware of a significant change in its activity.
Scott said experts were out near the volcano yesterday afternoon and saw ash falling within 1km of the volcano, with the plume drifting off to sea.
MetService meteorologist Mathapelo Makgabutlane said the wind direction had been mostly pushing ash away from the mainland but that was not the case overnight.
A northerly wind saw ash pushed towards the eastern Bay of Plenty, and high levels of that begin to pose a risk to aviation.
Makgabutlane said its advisory centre kept an eye on this and was in communication with the aviation industry.
Ash cloud up to 1200 metres above sea level was observed.
Emergency authorities said people do not need to be concerned about the volcanic activity.
Bay of Plenty Emergency Management's Lisa Glass said the volcano is going through an extended active period, but there was nothing particularly different happening there today.
She said it was incredibly unlikely that ash would reach the mainland.
Anyone out on the water should stay two nautical miles from the island.
On August 9, the the Volcanic Alert Level was increased to Level 3 and the Aviation Colour Code raised to orange following a minor eruption.
In 2019, 22 people died and 25 others were injured when the volcano - a popular tourist site - erupted.
