There is a week to go before the NDP leadership convention kicks off and 25,000 New Democrats have already made their pick using advance ballots, CBC News has learned.

There are about 131,000 NDP members eligible to vote for the new leader, meaning more than 80 per cent of the membership has yet to vote. The convention is being held in Toronto on March 23 and 24, but advance voting began on March 1.

Members can mail in a ballot or vote online with their assigned personal identification number. The advance ballots are preferential, meaning members rank their picks among the seven candidates. The voter's first choice is counted on the day of the convention in each round of voting as long as that person is on the ballot. Once an advanced voter's first pick drops off the ballot, their next choice still in the race receives the vote, then subsequent choices if necessary.

Voting on the day of the convention is not preferential; members will vote for one name, one round at a time. The person with the lowest number of votes is dropped in each round. Voting continues until one person has more than 50 per cent.

How many people vote in advance will determine to what extent the winner will already be decided before the convention because of the preferential system.

Mail-in ballot deadline extended

Members originally had until next Monday to mail in their ballots, but that deadline has been extended so that ballots received up until March 22 will be counted. Advance online voting closes at 9 a.m. on Friday, the first day of the convention.

A spokesman for Nathan Cullen's campaign said the 25,000 number doesn't mean much at this point.

"The figure that counts is next Thursday," Jamey Heath said. He said he thinks some people haven't voted yet because they are still mulling over who to mark down as their second and third choices.

There are seven candidates left in the race to replace Jack Layton, who died of cancer in August: Niki Ashton, Cullen, Paul Dewar, Thomas Mulcair, Peggy Nash, Martin Singh and Brian Topp.

There were several developments this week that made headlines.

On Thursday, for example, Singh revealed he is facing a fine of $1,100 for calling Topp a liar during one of the debates and he appealed to NDP members for donations to pay it. Earlier in the week, Singh said Mulcair is his second choice in the race, and said he would urge his supporters to go to Mulcair if necessary.

At the same time, former NDP leader Ed Broadbent gave several media interviews in which he expressed concerns about what would happen to the party if Mulcair won. He passionately defended Topp, who he has backed from the beginning, whom he said would keep the party true to its social democratic roots.