Senior National MP Amy Adams is the third National MP to make a bid for the party's leadership.
Ms Collins confirmed her bid this morning, followed by Simon Bridges before lunch. Mark Mitchell, who became an MP in 2011, said he was considering a run for the top job.
The party's deputy leader, Paula Bennett, said she would stand again as deputy, but not for the position of leader.
Amy Adams now looks set to join the race.
Follow RNZ's live coverage: