Mr Fletcher was appointed in late 2011 following a panel interview by head of the Department of the Prime Minister and Cabinet Sir Maarten Wevers, Defence Secretary John McKinnon and deputy State Services Commissioner Helene Quilter.
Mr Rennie confirmed to Fairfax yesterday that he had rejected the shortlist drawn up by a recruitment agency and that the panel was unanimous in finding Mr Fletcher suitable for the appointment.
Labour's deputy leader Grant Robertson said today that Mr Key's intervention in the appointment process reeked of cronyism and was further evidence of a disturbing lack of transparency.
"This looks like a jack-up to get John Key's mate appointed as our top spy. He was not even an applicant for the job, and ended up being the only person interviewed. The public of New Zealand deserve far better than this kind of cronyism, especially in a sensitive position such as this.
"Are John Key and Iain Rennie really trying to suggest that Mr Fletcher was the only person who deserved to be interviewed? And why was the position not re-advertised if the shortlist of applicants was rejected? That's certainly what would normally happen in those circumstances."
While Mr Key had told Parliament the appointment was made by the State Services Commissioner, it was very clear the Prime Minister's role in the appointment was significantly greater than he was letting on, Mr Robertson said.
Greens co-leader Russel Norman said there was no external scrutiny of the relationship between the head of the GCSB and the minister responsible, the Prime Minister, and that made it a particularly sensitive position.
"Normally you can have a bit of a look at the relationship between a head of a department and a minister because you can OIA the communications. We can't do any of that because it's all secret so it does make it a particularly questionable approach when you really want everything to be above board," he told TV3's Firstline programme.
"Of course it all became more complicated when all the illegal activities of the GCSB became known. Now that we know that the GCSB conducts illegal activities is it really the best situation to have someone so close to the minister as head of that department and that's really where the question lies now I think."