"The foundation does a fabulous job, and I'm delighted to lend our support to this pyjama fundraiser," Alexander said.
"It's confronting to think so many Kiwi kids might be going to bed cold this winter — but that's the reality.
"Please join us to ensure thousands more can go to bed snug and warm this winter."
Among those to receive pyjamas are Jackson, 3, D'Angelo, 19 months, and Giselle Iwihora, 9, seen above with their mum Angela.
Every winter, admissions to Kidz First, at Middlemore Hospital, skyrocket, from around 320 in January to 700 in August.
Many of those admissions are for illnesses that could have been prevented if kids lived in warm, dry houses, as well as having warm pyjamas.
Pyjamas collected during Jammies in June will be given to Kidz First patients, as well as distributed by the Middlemore Foundation to families in need in South Auckland.
Jammies in June started in 2013, with 4,200 pairs of pyjamas being donated, along with $17,800 in cash.
Last year, 5,400 pair of pyjamas went to kids in need, while cash donations topped $20,000.