Labor has called for an investigation into a lobbying firm run by National Party president Larry Anthony regarding a "clear conflict of interest" while the Greens have urged for a major overhaul of the rules governing lobbying.

Fairfax Media revealed on Saturday that Mr Anthony's firm, SAS Consulting Group, was representing energy companies such as Santos and Delta Electricity as the Coalition mulls important energy policy decisions.

Former MP Larry Anthony during National's 2015 federal conference in Canberra. Credit:Alex Ellinghausen

Mr Anthony's firm, which he founded, co-owns and manages as executive director, promotes him as a president of the Nationals, "one half of the ruling Coalition government". As president, he sits on policy and campaign committees, chairs the national conference and acts as a director of the Nationals' fundraising company.

But the former Howard government minister who lost his NSW seat in 2004 said he was "not directly a lobbyist" and so did not need to appear on the federal register, managed by the Department of Prime Minister and Cabinet, which he removed himself from in 2015.