NSW Labor will send almost 20,000 voting papers to members from next week as the party runs a 'historic" membership ballot to elect a new leader.

The caucus executive on Tuesday voted to open nominations for NSW leader on Wednesday, closing at 2pm on Friday. Kogarah MP Chris Minns and Strathfield MP Jodi McKay are the likely contenders.

NSW Labor frontrunners: Jodie Mckay and Chris Minns. Credit:Kate Geraghty, Janie Barrett

To nominate, a candidate will need 15 signatures on their nomination form from caucus members.

It is expected the ballot will take at least five weeks, and is likely to cross-over with the federal ballot to elect a new leader to replace Bill Shorten, who will stand down after his shock election loss.