Collins' resignation is the biggest scalp yet for the Dirty Politics scandal, although rival politicians were manouevring to claim credit for her undoing.
A National Party insider said the priority was now "saving face" and not making Collins' resignation appear as a victory for rival parties.
At a press conference yesterday, Collins said she'd been "the subject of a two-year smear campaign".
She said she was shown contents of an email between blogger Cameron Slater and others. The email suggested Collins was trying to remove Serious Fraud Office boss Adam Feeley.
Collins rejected any suggestion she was gunning for Feeley, who had been an "excellent chief executive".
Collins said she asked the Prime Minister to convene an enquiry and promised to co-operate with the investigation.
She said her resignation would allow John Key to focus on the election. "This has been an extremely distressing and disappointing time," she added.
About 18 of Collins' friends gathered at the National Party offices in Epsom, where campaign banners and messages of support lined the walls.
Collins walked off after reading her statement, refusing to answer questions.
