Big business.
Which is why, at the risk of sounding like a broken record, we should outlaw all political donations. Everything. To anyone. Not a dollar to any political group.
It wouldn't be rocket science for the AEC to work out and deliver an advertising & staff salary budget to every political party and Independent, based on a fair formula and kicking in under certain conditions.
That way we all pay the political parties through the tax system, so there's no incentive for them to listen to lobby groups.
To put it simply, he who pays the piper calls the tune. So if we sit back and let the Minerals Council of Australia or the Australian Christian Lobby give money to politicians instead of us doing so, we shouldn't whinge when the pollies listen to them.