Police are investigating after a toddler was allegedly punched in the face by a stranger in an Auckland supermarket during the weekend.

Father Shane Maddren shared in a Facebook post that 18-month-old Theodore "took it like the champ he is" when an unknown man hit him in the side of his head, while they were waiting to be served at the Wairau Pak'nSave on Sunday morning.

Mum Shelly Maddren told Newshub two men were in front of them at the deli, one in his 20s and the other she believes was his father or carer, when the younger man began getting "a bit irritated".

"The next thing I knew he had turned around, I think in frustration or maybe being little bit overwhelmed in the situation he was in, and just lashed out," she said.

"It just happened that we were right next to him and with a closed fist, [he] just punched my son Theo in the side of the head."

Ms Maddren said the older man immediately grabbed the younger and "removed him from the situation", not saying a word to her and leaving her stunned. The family believe the man may have been intellectually disabled.

As Mr Maddren said in his Facebook post, Ms Maddren said her son didn't cry and was more surprised by the sudden assault.

"I think he was more a bit confused about what had happened and, 'My head hurts and I'm not sure why', and seeing the boy getting carried off I think he was just a bit concerned."

Theo began vomiting later that afternoon and spent the night in Starship Hospital. Doctors weren't sure if it was due to the punch or an unrelated viral infection, Ms Maddern said.

"He's a little bit clingy [at the moment] and a usual toddler, but he's been pretty good today," she said.

With her son in good health, her concern is now turned to the man who hit the young boy.

"We don't hold any malice to the boy and his father or carer," she said.

"We just really want to... make sure that he's got support for him, because I'm sure his carer will be feeling awful about what happened, and if the [younger man] can comprehend what happened as well he might be feeling really bad as well."

She hopes something similar won't happen to another family.

A police spokesperson confirmed to Newshub they're investigating a "report of this nature", after an 18-month-old was assaulted by an unknown main inside a Wairau Rd supermarket.

Newshub.