Temperatures would likely be 9-11C in Auckland and Wellington, while Christchurch and Dunedin should be a cooler 8C.
Mr Corbett recommended wrapping up with an extra layer.
"That fresher breeze from the west will make it feel perhaps coolish - and it may perhaps be damp underfoot as well, even if it's not raining at the time, from some of those overnight showers in places.''
RSA national president Don McIver said there would likely be strong numbers at dawn services throughout the country tomorrow, regardless of the weather.
"Most of the people I speak to say, 'If I get wet, I get wet.'"
Mr McIver said the upcoming centenary of World War I would be in many people's minds tomorrow, as would the withdrawal of the last New Zealand troops from Afghanistan this month.
RSA national vice president David Moloney said crowds at dawn services had been increasing each year, with more young families in particular.
"I'm heartened by the numbers that are going to dawn services and the increase in interest in what our very good servicemen have done, starting in the Boer War and going right through to the current day."
He said the weather could put some people off.
"But my experience in the past is that more often than not, it's cold and rainy on Anzac Day, so it's part of the folklore."
Mr Moloney said the weather added to the solemn nature of the occasion.
"It's a chance to think about your relatives or indeed, in a number of our cases, our former colleagues who are no longer with us."
Commemorative parades and wreath laying ceremonies will be held throughout the country tomorrow morning.
The national commemorative service will be held at the National War Memorial in Wellington, which will also host a dawn to dusk vigil at the Tomb of the Unknown Warrior.
Auckland Domain: 5.30am
Hamilton Cenotaph: 6am
Wellington Cenotaph: 5.30am
Christchurch, Cramner Square: 6am
Dunedin, Queens Gardens: 6.30am
* A full list of Anzac Day services can be found here.
* Full Anzac Day coverage can be found here.