Work and Income is vowing to investigate people who are trying to sell their WINZ cards on Facebook trading groups.

In a screen-shot provided to NZME, one person has put their $100 payment card up for sale for just $40.

WATCH: Ministry of Social Development associate deputy chief executive Liz Jones speaks with Jack Tame

On another post, a woman says she had to put her card up for sale as she needs money for petrol.

Ministry of Social Development associate deputy chief executive Liz Jones says she'll be following up on those cases.

"If we can prove anything, we would take action," she said.

"Obviously if people have money that they have no entitlement to, they have to pay that back."

Jones said that if anyone is worried about their situation and needs help with essential needs, they should give Work and Income a call.