Surprised it is the same with SRI in Vic it doesn't have to be a Christian based church it can be any recognised religion that approaches the school wanting to teach their religion.
That is not how I understand it.

Here is the info from the Vic education website:
Accredited and approved instructors

Only accredited instructors, approved by the Minister for Education may deliver SRI in Victorian government schools.

Accreditation includes a Working with Children Check, registration, training, minimum suitability standards for persons who work or volunteer with children, and Ministerial approval.

The Department engages two major organisations to administer the accreditation and approval of religious instructors. They are (1) ACCESS Ministries, and (2) World Conference of Religions for Peace.

ACCESS Ministries’ religious instructors deliver Religion in Life® Christian Religious Education, which is an agreed syllabus representing 12 Christian denominations. Students receiving such instruction are not to be grouped for SRI according to denomination, but taught in their usual classes.

The World Conference of Religions for Peace (a non-government organisation associated with the United Nations to interface with the world’s faith communities), currently administers the accreditation and approval process for religious instructors in other recognised faiths including Bahá’Ã*, Buddhism, Sikhism, Hinduism and Orthodox Christianity.

The Catholic Education Office/Diocese and United Jewish Education Board also accredit religious instructors in their respective faiths.

Note: When parents request SRI in a faith that is not available at the school, the principal should advise them that such instruction can only be scheduled if the particular religious group provides an accredited instructor who is approved by the Minister for Education.

Parents may withdraw consent for their child’s participation in SRI by notifying the principal in writing. It is not necessary to renew or withdraw consent each year.

At the start of each year ACCESS Ministries provides schools with the names of accredited instructors allocated to them. Schools are also informed of changes to instructors during the year. This information should be retained on file at the school.
As Nai as said, Volunteers mostly come via Access Ministries, so one view of Christianity.

There is also the issue that not all schools have been 100% clear in the change to Opt In and what it is about. I know I had to greatly question the school, as at the time they still referred to it a RE. Even now, the odd communication may say SRI, but a lot of the time they fall back to using RE and just confusing the situation.

So if children opt-out what do they do during the time that is spent being taught the SRI?
From the site:
ensure that students who do not attend SRI are appropriately supervised by teachers, and engaged in positive, independent learning such as self-study, including revision or other activities, for example, community service, peer mentoring, participation in clubs or instruction in areas outside the core curriculum
ensure core curriculum is not delivered to non-attendees during the period of SRI
Depends on the school. Some kids still sit in the room, but up the back. Sometimes on computers with headphones. At our school, it depends on the grade. The foundation students move across to the other foundation room. The older grades seem to go into a side room, with windows, so the teacher "supervises" them whilst still staying in the room with the volunteer. When my eldest was first enrolled I spoke to both the principal and the teacher to find out what they did and to point out that to us "opt out" does not mean in the same room.

I have read of students being sat in hallways, but this was before changes were made in relation to what they were allowed to do, so not sure if it happens now.