"I don't see it getting very settled until about January 16 and 17 - but even then it's not going to be perfect," Mr Duncan said.
Showers and rain are forecast for most of next week in areas including Auckland, Hamilton, Taupo and in and around Northland.
A low covering most of the North Island is expected to clear by about Tuesday, but another low is forecast for the week after.
"We haven't seen any of those blue-sky summer days where the grass is dry, and it doesn't look like it's going to change any time soon," Mr Duncan said.
The bad weather has sent some campers and holidaymakers home early, escaping heavy rain and patchy weather in popular holiday spots such as Northland, the Coromandel and Gisborne.
Geoff Hawthorn, owner of the Dickson Holiday Park in Thames, said some campers had left early, and others had postponed their bookings.
"We had a lot cancelled during that Christmas-New Year period. That week between Christmas and New Year wasn't really a week to be in a tent in the Coromandel.
"It's not a great summer from a business point of view to be a camping ground in these areas."
At the Hahei Holiday Resort on the Coromandel Peninsula, the weather has also forced campers out.
Director Ian Carter did not know the exact number that had left, but said it was "noticeable for the area".
If you are unlucky enough to be back at work already and feeling a little ripped off at a rained-out holiday, don't be too glum, says one psychologist.
Being optimistic and planning for next year's holiday could be a way of keeping the rainy-day blues at bay, says University of Waikato professor of psychology Michael O'Driscoll.
"It's probably a good idea for people to say, 'Okay, this one didn't quite work out but let's think ahead and plan something else for the next time'.
"Don't dwell too much on what's happened and instead think more optimistically about a future vacation."
Professor O'Driscoll said the weather would be a disappointment to families, especially those who had forfeited their holidays and left the beach early, but it was out of anybody's control and getting depressed about it would not help anyone.
"It's hard for me to imagine that anyone would be enormously depressed. What people need to do is put everything into perspective and not over-emphasise the weather.
"My feeling is you really can't do a whole lot about it and you've got to make the best of it, and if things don't work out quite according to plan then it's a good idea to have a plan B."
Even if the weather had been brilliant, Professor O'Driscoll said, evidence suggested the happiness of being on holiday didn't last long after a person returned to work anyway.
He said one study showed that after people had been back at work for three days their stress levels were back to where they had been previously.
"Obviously we do need holidays. We need to recharge the batteries and recuperate but quite often the long-term effect can be quite small on people's overall wellbeing."
Professor O'Driscoll said February was the best month of summer and it was a good time to go on holiday.
"It's when everybody else is back at work. You can be more sure of what the weather's going to be and for people without kids it's an advantage to go away at that time."
