Jerusalem Prosecution witnesses from a private investigation into the mental health of alleged child sex offender Malka Leifer will have to deliver evidence in writing, the Jerusalem District Court has ruled.

Leifer's case has been delayed again with the next hearing is set for May 15.

Accused child sex abuser Malka Leifer (right) pictured in February last year. Credit:AP

Her extradition trial is determining whether the former principal of an ultra-Orthodox Jewish girls' school in Melbourne should be returned to Australia to face court on 74 charges of child sex abuse.

A private investigation carried out in December 2017 provided video footage countering the defence's claim that Leifer is mentally unfit to stand trial, instead showing she was fully functional in her daily life.