Labour's primary industry spokesperson Damien O'Connor has questioned the lack of a breakdown of income in Federated Farmers' financial statements, but the group said the revenue could all be explained.
Separate to its membership fees, each year about $1 million is declared in Federated Farmers financial statements as "other revenue" and is not broken down further.
Mr O'Connor said the federation's 'other revenue' added up to about $7 million since 2008.
He was concerned it may include money from behind the scenes organisations trying to influence Federated Farmers' policy and positions on issues.
"Well it is concerning that there's such a large amount of money unaccounted for or unexplained, and Federated Farmers is a long standing and highly respected organisation. I would hope they would be able to explain to all their members exactly where this money might be coming from and what it's used for, so that the members are confident that there's no untoward influence from that money."
However, Federated Farmers said that was not the case.
It said the other revenue was mostly made up of income from its range of employment contracts and agreements it sells to farmers, the leadership courses it runs, as well as its formal business partnerships.
And it pointed out it has not been afraid to lock horns with business partners, like Transpower, on behalf of farmers.
A former chief executive Tony St Clair said Federated Farmers was a very wealthy and iconic organisation and the "other revenue" money could be coming from a range of sources - including its branches, bequests, donations or possibly even its farmer fighting fund.
He said he did not believe the farmer lobby group would run a cash for influence scheme.
"They guard their apolitical stance on all these issues very, very much and as such I'd find that hard to believe, but in saying that, I wouldn't under estimate that there are parties that would like to contribute."
Mr St Clair said it was up to Federated Farmers how much detail it disclosed in its financial reporting.