He told the Herald yesterday he had dropped from 73kg to 60kg. He said he was campaigning for beneficiaries' families, and children in particular.
In his letter to Ms Bennett, Mr Kuha asks for a meeting with the politician and extra support for people with disabilities.
"Please remove stalling policies around food grants, give food where and when it's needed," he said. "Don't make families jump through hoops to get food."
If an effort was made towards a remedy, Mr Kuha said he would would start eating again.
Ms Bennett said: "I respect the right of any New Zealander to protest and I think that Sam Kuha has made his point loud and clear. Now I'm more concerned that he eat and not make himself unwell."
Lance O'Sullivan, who has spoken out about poverty in the Far North, has been checking on Mr Kuha. He said people could last for weeks without food and while Mr Kuha was not eating solids he was keeping up fluids and taking teaspoons of honey.
"He's going into the red zone and he's upping the ante now that he's gone on for over three weeks. He's losing reasonable weight so I guess now we'll see some changes in his health."
Mr Kuha's failure to receive a food grant was not a rare occurrence in the region, Dr O'Sullivan said.
"I think this is happening to many of our families and I think that's wrong. We have the most desperate people in our country accessing a service which has no regard for their dignity."
Labour MP Jacinda Ardern released Government figures which showed that in four years the number of hardship grants for food nearly doubled. In the 2007/08 year 285,460 food grants were approved. In 2010/11 the number was 553,854 - an increase of 94 per cent.
Ms Ardern said the numbers were worrying.
Excerpts from the letter
"Dear Ms Bennett,
I am a former pipe-line worker from Kaikohe. I have been in a wheel chair for twenty years. I try hard to remain independent but the increases in what I consider unnecessary demands placed on all beneficiaries leaves ordinary people whoare already in a poverty- stricken state, worse off than ever before ...
"I've had to go to extraordinary lengths on this because nothing gets sorted ... and people keep telling us there's nothing that they can do.
"As the minister in charge of WINZ I believe you have an obligation to get directly involved to resolve these problems."