Activity at White Island is increasing, but tourism operators are still taking tourists to the volcano.
GNS raised alert levels at the volcano on Thursday after unsettled activity indicated White Island could erupt at any moment.
GNS head of volcanology Gill Jolly said the level of volcanic tremor had started to decrease, but hybrid volcanic earthquakes had appeared.
She said hybrid earthquakes were typically associated with the movement of magma within an active volcano.
This change in activity was consistent with an increase in the level of unrest so GNS upgraded the Aviation Colour Code to Orange.
"Visitors to White Island are now at the highest level of risk since the start of the 2012 eruptions.
"Hazards to visitors can include the health effects of volcanic gas exposure, including respiratory issues, skin and eye sensitivity to acid gases.
"Explosive eruptions can occur at any time with little or no warning. We advise a high level of caution should be taken if visiting the island."
Discussing potential scenarios, Dr Jolly said the unrest could result in explosive activity, extrusion at the volcano's lava dome, or nothing at all.
For White Island Tours, this situation is nothing out of the ordinary.
Marketing manager Patrick O'Sullivan said the company had been in operation for 20 years and had seen activity similar to this in the late 1990s and early 2000s.
"At the moment, we are using an advance scout, basically someone who goes ahead of the tourist group to scan out the paths we follow to check they're safe before the groups get there.
"We have also adjusted our tracks, we're not going as close to the main crater as we usually would and keep in contact with GNS and also with Civil Defence and other operators."
Mr O'Sullivan said White Island Tours had visited the island within hours of the August eruption.
"They tend to be a one-off event, not ongoing as such.
"Even when there was a large scale eruption in 2000 we were out there in a couple of days.
"The tourists going out there at the moment are coming back buzzing, it's really impressive."
Rotorua-based Volcanic Air Safaris is continuing its tourist operations over the island but owner Dorien Vroom said they were keeping a close watch on conditions.
"We're watching it closely and keeping in close contact with GNS and other operators.
"We're still heading out there but we're taking high precautions."
Ms Vroom said they were also reiterating to their groups there was the potential they might not land on the island.
- Additional reporting APNZ