Just like when Mt Ruapehu erupted in the 90s wind direction will be everything. It depends on the amount of ash ejected, and how far up into the atmosphere it went, the wind direction can blow the ash in any direction. When Mt Ruapehu erupted ash fell as far north as Auckland. However the prevailing wind may be our biggest helper.
"The prevailing wind in New Zealand is a westerly - and this is never more true than as we head into September" says head weather analyst Philip Duncan. "This means the general prediction is that ash will travel eastwards over more remote parts of the country".
Of course, not every day will have westerlies - and for the past few weeks northern New Zealand has been dominated by winds from the east and north east.
Overnight and today
WeatherWatch.co.nz says the wind direction overnight was a westerly and throughout today the winds will turn more north westerly as a front and large low approaches.
"This will blow any ash east to south eastwards towards the Desert Road and other unpopulated areas".
Mr Duncan says ash would cause far more issues under an southerly - which would blow ash north towards Waikato, Auckland and Bay of Plenty - or a westerly which would see ash blow across the main flying route in New Zealand south of Auckland and would mean significantly more flights re-routed..
"The winds don't look especially strong over the next few days as the centre of a low crosses the North Island - the lighter the winds are the more ash will fall locally around the mountain and less likely to cause widespread disruptions further afield".
Today's wind map shows building north to north west winds which should keep the ash around less populated parts of NZ.
WeatherWatch.co.nz's 4 Day wind prediction for ash cloud movement
• Today - Light north to north west winds (blowing ash over the Desert Road)
• Weds - Light north west winds dying out as centre of low moves in (blowing potential ash south east from the mountain)
• Thurs - Light winds, tilting more south west (any ash mostly localised)
• Fri - Light winds