The Problem Gambling Foundation will fight the loss of most of its funding at the High Court today.
The foundation lost the government contract for the lion's share of its services in March, when the Ministry of Health awarded it to the Salvation Army.
However, the foundation questioned that process and is now taking court action.
Foundation chief executive Graeme Ramsay wants the process re-examined.
"Our concern is that the process should be fair, robust and transparent, and not come out with strange results," he said.
"This one did and it led us to question the process itself, and we're going to court saying this wasn't right and the process should be redone."
Mr Ramsey said the Ministry of Health told him the decision was not about price or performance, so he was not sure what it was based on.
He said his lawyer, Mai Chen, has told him he had a good case.
"We're confident, and we're going in thinking that these processes need to be held to account - and we're about to do that," he said.
The Ministry has said the tender process in question was independently reviewed, and the change will mean more frontline staff to deal with addiction issues.
The case is being heard at the High Court in Auckland.