Nominations to replace David Shearer opened on Friday and will close on Monday night.
The Labour Party's wider membership had no say in David Shearer's appointment in 2011.
The rules have since changed, with members and affiliated unions also getting to vote alongside the caucus - as long as more than one MP is in the running.
Party president Moira Coatsworth said she would endorse the new leader if only one MP was nominated - but that wasn't her preference.
"There's a widespread view among members that this is an important process. Selecting the next prime minister, they welcome the opportunity for that to be a really robust, careful process - rather than a meeting behind closed doors in the caucus," she said.
Those most likely to throw their hats in the ring are, once again, David Cunliffe and the current deputy, Grant Robertson. Andrew Little has also refused to rule out making a bid.
The results will be announced on 15 September after a series of meetings nationwide.
Ms Coatsworth rejects suggestions the process might reopen divisions between the party and the caucus. Politicians were mature and would look at what was best for the country rather than who they may have friendships with.
She said she'd had confidence in Mr Shearer as leader but it was time for a change.
Senior Labour party MP Clayton Cosgrove said Mr Shearer was being courageous in standing aside and putting the cause of the party first.
The party, he said, should learn from that, stop talking about itself and start focussing on the people it represented.
Party general secretary Tim Barnett said contestants would be subject to a code of conduct to ensure a fair process.
The party council will meet again on Sunday to finalise the rules governing the election process.