A NSW Nationals MP verbally attacked a female cabinet minister, threatening to "tear her a new orifice" and saying she had "never had a real man", because he was angry at her department's decision.

The attack on then environment minister Robyn Parker was made by Murray-Darling MP John Williams in front of about 100 Nationals members during his bid for preselection for the upper house in April.

Ms Parker was raped as a teenager, and had spoken about her ordeal in parliament in 2004, recalling she nailed her bedroom windows shut in the aftermath.

Multiple sources who attended the Nationals preselection meeting on April 4 at Parliament House said they were offended by Mr Williams' comments. The event was filmed, but the slurs went uncensured by party leaders in the room, including deputy premier Andrew Stoner.

Mr Williams emerged as an endorsed candidate, albeit in the almost unwinnable fourth position.