Would a staff member at a rape crisis hotline be obligated to report past (not ongoing) sexual abuse of someone still under 18?

Thanks for the question, now remember this in no way is meant to substitute legal advice, if you need a lawyer contact one.

Mandatory reporting is when certain types of people must report to the government situations in which they reasonably believe a child is suffering sexual abuse.

The specific act of the WA parliament is: Children and Community Services Amendment (Reporting Sexual Abuse of Children) Act 2008, part of the Children and Community Services Act 2004.

The people that have to report sexual abuse are doctors, nurses, midwives, teachers, cops. These professionals have to have a reasonable belief that a child is being sexually abused.

Other abuse can be reported, like neglect for example, but there isn't a penalty for not reporting it. There is a penalty if they do not report sexual assault.

The people who are obliged to mandatorily report child sexual abuse, also have to report abuse that occurred in the past, but if it occurred before the law was put in place then there's no penalty for not reporting it.

So, it would appear that unless a person working on a rape crisis hotline is a nurse, midwife, teacher, doctor or cop they do not have to report the sexual abuse to the WA government.
For more information: http://www.mandatoryreporting.dcp.wa.gov.au