Teenagers could also see a fall in literacy over the end-of-year break.
"You've got those parents, you see them in the libraries and their kids are checking out 20 books at a time - they are the ones that are really going to move quickly and do well over summer," Professor Nicholson said.
"Then you see other families where they don't go to the library over the summer break, and their kids are just not getting that practice.
"So when they start school next year they are going to miss out big time, because they are going to be way behind those other kids."
This summer some students will go from daily reading at school to almost none.
Halting the summer slide can be as simple as encouraging kids to read five age-appropriate books over the holidays, regardless of genre.
Low-income families are mostly affected by holiday reading loss because of limited access to books and other learning materials.
This year libraries across the country are promoting a range of summer programmes in a bid to counter the phenomenon.
Alison Sutton, strategic analyst at Comet Auckland, an Auckland Council-controlled organisation and also an independent charity, said the learning gap between privileged kids and those from wealthier families widens significantly over summer.
Having access to books was a big problem, and the library programmes being offered were a way to counter that, Ms Sutton said.
Best-selling author Malcolm Gladwell, writing in his book Outliers, argued the length of the holiday break in North America was a major reason for the achievement gap between more well-off and poorer students.
Professor Nicholson said that, although our break was shorter, there had been similar calls here to change school terms to reduce it.
His advice to parents was to make sure their children kept practising their reading. Libraries had a huge range of books that would appeal to all kids, including graphic novels.
"Reading is not just something that's fun. It really will affect your whole career and your life - your income level, your socio-economic level, the sort of job you've got - whether it is interesting or really dull."
Family bookworm passes on the bug
The Wilson family: Rebecca (10), William (6), Caty Ferguson and Alexa (8). Photo / Nick Reed
Having an older sister who is a compulsive reader has meant Alexa and William Wilson often get taken along to the library whether they like it or not.
Luckily it has turned out Alexa, 8, and William, 6, have grown to appreciate those visits to Massey Library.
Their mother, Caty Ferguson, said William was mad about Star Wars and had discovered a range of books on the science fiction series which could be ordered through the library's website.
Alexa had taken to Harry Potter audio-books, but was the least keen reader of her children, Ms Ferguson said, and was initially reluctant to rejoin Massey Library's Dare to Explore holiday programme.
Run over the summer, the programme has participants keep a reading log but also complete related activities and challenges, including creating new games and a potion or experiment plan.
When her eldest child Rebecca, 10, and other neighbourhood children began writing "invisible" messages in lemon juice as part of the programme, Alexa overcame her reluctance and was now on board, Ms Ferguson said.
After last year's reading programme, the librarian had commented on a real change in Alexa's reading ability.
"Around that age they change from that early learning-to-read stage to reading for meaning or to get information and pleasure," Ms Ferguson said.
"And I think Alexa just did that over that summer break."
