Commercial nursery owners are trying to stop the Department of Corrections selling plants grown in prison training programmes.
About 20 growers travelled to Hamilton for a meeting on Friday with two Corrections representatives.
The owners estimate the prison programmes sell as many plants as two or three medium-sized nurseries, and are able to undercut other operators because inmates are paid so little.
Inmates can be paid between 20 cents and 60 cents an hour for their garden work, compared to $20 an hour for workers in commercial nurseries.
The Nursery and Garden Industry Association says some growers have gone out of business because of the prison sales.
The association chief executive, John Liddle, says the industry supports prisoner training but Corrections has taken it a step too far.
"There is a fine line between a training nursery which will obviously produce some commercial product and a nursery which is indeed producing commercial volumes into a commercial market," he says.
Mr Liddle says growers at Friday's meeting directed him to work with the Department of Corrections to have its sales stopped.
The acting head of the inmate employment programme, Ruth Turner, says the department is looking at what it can do to address grower concerns but it's too early to say what changes it will make.
She says the programme is aimed at training prisoners properly in order that they can join the industry when they get out.
"The training and the qualifications that we're able to get them need to reflect working in a proper commercial environment, so there's certainly an element of us needing to provide that within the things that we do."