John Robertson has resigned as NSW Labor leader just hours after moves began to dump him over revelations he signed a letter of request for Lindt siege gunman Man Haron Monis.

Mr Robertson's decision to quit came after senior colleagues made him aware they had planned to visit him at 5pm on Tuesday in a bid to resolve the leadership issue.

The decision means the ALP will elect a new party leader only three months before the March 2015 state election. The new leader is likely to be chosen by a ballot.

Shadow Treasurer Michael Daley and environment spokesman Luke Foley are being discussed as leadership candidates.