Tour companies have added extra guides to their tours when visiting White Island.
GNS has raised the threat level of an eruption to a level two - meaning it could potentially happen.
From the air, it's hard to miss the vivid white steam and gas seen rising off the volcano.
"It's been quite a while since we've had this level of unrest at the island," GNS volcanologist Brad Scott told Newshub.
Scientists raised the alarm when they found almost 1900 tons of sulphur dioxide pouring out - three times the normal daily amount.
"White Island is an active volcano, so there's always the possibility of eruption," Scott says. "And when parameters increase, that likelihood has also, by definition."
White Island is the country's most active volcano. It has a reputation for being notoriously unpredictable.
It last erupted in 2016, but being 48km offshore there's no serious concern for locals.
However it's popular with tourists, who every day are ferried to the island on tours.
"Over the last few years since 2013 there has been some levels of heightened activity and some small eruptions," White Island Tours general manager Patrick O'Sullivan says.
"Until we're quite familiar with working with it and on it, during those situations, we'll have an additional staff member who will scout the island ahead of any tour groups."
Scientists will be keeping a close eye too, flying over it tomorrow to check if it's any closer to erupting.