A woman's heartfelt offer to pay for an elderly man's groceries has gone viral and melted the hearts of the nation.

Aneta Mitai was in Pak'n Save, Papamoa, with her 2-year-old autistic son when an elderly man stood and entertained her son, a gesture that struck a chord with her.

"I was at the checkout with my son, Sirus, who was starting to become restless and kicking off his shoe and hat when the elderly man approached him and put his shoe back on," Mitai told NZME.

"He started talking with my son which took his attention away from climbing out of the trolley.

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"He even told him to put his potai [hat] back on because it's very cold in here, which blew me away since he acknowledged our Maori culture."

Mitai praised the man, saying his kind nature towards her son inspired her to then pay it forward to the man.

"It meant a lot for me as a mother with an autistic toddler for a stranger to show affection towards him as we have had previous unpleasant encounters shopping with people rudely commenting on my disruptive, naughty child.

"His kind gesture made the shopping experience that bit easier, so I decided to repay his kindness."

Mitai then approached the man, and told him she'd like to pay for his groceries.

His reaction: priceless.

"No way! That's a fortune there! I can't let you do that," he told Mitai.

However, Mitai insisted the man let her pay.

"You picked my son's shoe up and you were entertaining him, so that was awesome," she told the man.

"Whatever it is, I'll pay for it."

The man then told Mitai he was "very grateful" for her kind gesture before the pair leaned in for a hug.

A video of the heartfelt moment was posted to Facebook on Monday and has had more than 39,000 views and 1000 reactions.

Hours after the video was posted Mitai received a message from a member of the public who knew the man in the video.

"Hey, I've known Ian [guy in your video] almost all my life and you honestly have done the best thing possible to one of the nicest most kindest guys I've ever known. You are awesome! You've made my week, and it's only Monday," the message read.

Hundreds of people have praised Mitai and the man for their kind spirits.

"Paying it forward has nothing to do with if the person deserves it or not, it's the heart of giving and the art of receiving. All human spirit needs to be uplifted with kindness," one person wrote.

"Such an awesome thing to do. Would've made his day and there would have been plenty of positive ripple effects that many would have felt from this one selfless, kind act," another said.

Mitai told NZME she hopes others are inspired to pay it forward.