Melania Osborne had a bike for her son paid off by the mystery good Samaritan.

There's a Secret Santa making the rounds in Nelson.

An anonymous local woman has paid off $300 worth of strangers' lay-by purchases at bike shop Crank House.

It's the second year she has done it, refusing any recognition as she goes about her random acts of kindness.

MARTIN DE RUYTER/FAIRFAX NZ Melania Osborne with Crank House owner Glen Ogle.

Melania Osborne was one of the recipients of the mystery woman's generosity this year.

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The single mum had been paying off a bike, the main Christmas present for her 3-year-old son Jordan, since October.

With $30 left to pay, Osborne got a phone call from Crank House on Monday morning.

She thought the manager was lying when he said the bike had been paid for in full and was ready for pick up.

"I thought something was wrong, I thought someone was joking," she said.

But once she realised the kindness of a stranger had eased her financial burden over Christmas, she was "gobsmacked".

"It was really, really cool," she said.

"Obviously the money is a big thing, but it's just also the generosity of it and the fact someone is out there doing that for people."

Osborne works part-time and said an extra $30 in her pocket was a lot of money to her little family.

She wanted to reach out to her benefactor to say a big thank you from her and her son, and wish the woman a merry Christmas.

"Somebody's out there, thinking of other people at this time of year and changing their weeks," she said.

"I'm just thinking about how many other people she's affected; it will literally change some people's Christmas for them."

Glen Ogle, the owner of Crank House, said the woman didn't seem to be particularly well off or extravagant.

The long-time customer simply wanted to do something for others.

She had come in earlier in the week with the money, and asked for it to be split between customers who still had outstanding payments.

Seven people's purchases had been paid off from her donation.

"It's heartwarming," Ogle said.

"It's restoring my faith in humanity, especially after everything that's happened this year. It's nice to see someone finding the good in the world."