Hundreds of cancer patients are being placed on the Jobseeker benefit while they are getting treatment.
The Cancer Society calls the system ludicrous and says it forces people with the disease to jump through hoops to pay their bills.
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One woman, who does not want to be identified, applied for a benefit when she was diagnosed with breast cancer.
She was put on Jobseeker Support, which replaced the sickness benefit after the 2013 welfare reforms.
She said she had to pay for a medical certificate every month to prove she could not work - even though her surgeon insisted she would be off for much longer.
"The letter from the hospital wasn't sufficient. I then had to go back and get a doctor's note to keep them happy, just to prove the fact that I was going in for surgery," she said.
"Then I also had to, on the day of my surgery, get someone from the hospital to fax through that I had been operated on."
Cancer Society chief executive Claire Austin said the woman's story was common, and the system lacked common sense and sensitivity.
She said many cancer patients had never been on a benefit before, and deserved help while they were going through an extremely tough time.
"The situation really is ludicrous. We've got people who are already in work, who are unable to work because they are either sick and have to go through treatment, or have surgery.
"They have to then apply for a benefit, which is a benefit that requires them... to be available to work."
Exemptions not enough - Cancer Society
Jobseekers can be made exempt from work obligations if they are sick, and figures from the end of June showed 806 beneficiaries with cancer had a work exemption.
In total, 118,072 people received Jobseeker Support, and 54,817 had reduced work obligations due to sickness or disability.
But Ms Austin said, while cancer patients did not have to attend job seminars and interviews, it was still too hard to get and maintain financial assistance.
"People have to go through appointments, they have to go through the various forms. Then they have to get the medical information as well. I mean really, it should be made very simple. It could be made so much simpler."
The Ministry of Social Development cannot comment on specific cases without the person signing a privacy waiver.
But it said it knew illnesses could stop people working, which was why the exemptions were in place. It also said it took its advice from doctors.
Minister of Social Development Anne Tolley acknowledged that having to provide monthly medical certificates in the early stages of cancer was difficult, but said the government had to draw a line somewhere.
She said if cancer patients were given special consideration, other people would want those considerations as well.
"Where you draw the line is always the issue," she said.
"You start creating a whole lot of layers and there would be, I'm sure, other groups of people that would come forward and say, 'we need special consideration too'.
"What I have asked MSD to do is to sit down with the Cancer Society and work through this, [and] see if there are some things we can do to streamline it."
The Green Party said the government can, and should, make cancer sufferers eligible for a Supported Living Payment.
Green MP Jan Logie said Ms Tolley could do much more than arrange a meeting with the Cancer Society.
She said people with cancer were battling the system, just to keep a roof over their head.
Labour Party spokesperson Carmel Sepuloni said the government needed to expand the Supported Living Payment to people with cancer - not just the permanently disabled and terminally ill.
"People, and I've been told this anecdotally, are getting letters saying what are they doing to try and increase their income - otherwise known as trying to get a job," she said.
"Many of these people are just trying to get through the next few months, are trying to get through the cancer, [and] they don't need the additional pressure of trying to go out and find employment."