GNS Science has changed the volcano's code from a "normal, non- eruptive state" to "experiencing signs of elevated unrest above known background levels".
The ash advisory system for New Zealand assigns a colour code to volcanoes, giving the aviation industry a quick indication of the ash hazard for aircraft.
"The code has changed just to let the helicopters and fixed wing aircraft landing on White Island know that something has changed that could increase the possibility of something getting in the way of aircraft," Mr Rosenberg said.
The volcano's lake level quickly rose by about 3m to 5m some time between Friday and Saturday last week, exposing a "vigorous" flow of gas and steam into the air.
It has risen in the past, but took much longer than the 24 hours it took to rise last week, according to GNS.
During the past few weeks there had also been some minor volcanic tremor, including several hours on Saturday, Monday and Tuesday.
Last year and earlier this year the lake started to evaporate and exposed steam vents and two large muddy pools.
"We were all wondering whether the crater would completely dry out and start producing ash but it appears it has sorted itself out," Mr Rosenberg said.
"These phenomena are typical for White Island's activity, but are the first substantial changes to occur in the last few years.
"I think we have to remember that White Island is a natural system and it often balances things out on its own. We can monitor and model all we like but it [the volcano] will do what it's going to do and all we can do is try and understand and inform people of the changes."
The last time White Island was in the news was in 2010, when tour operators were sticking to high spots on the volcano as its crater lake came close to overflowing.
Because there had been no eruptions on the island since 2000, the crater lake had continued to cool and fill and was only 4m from spilling.
