Former foreign minister Julie Bishop has questioned whether some Liberal MPs acted illegally during last month's leadership spill.

Defending the Liberal Party's handling of bullying complaints by female MPs, Ms Bishop said the party is doing the right thing handling bullying complaints internally instead of in the public eye.

Ms Bishop also backed fellow Liberal Julia Banks, who is quitting politics over "appalling" bullying, intimidation and harassment during the recent leadership spill.

"It's a very difficult situation where you are accusing others of behaviour that could in some instances be illegal," Ms Bishop told reporters on Thursday.