Hi guys,
Thanks for your responses. Thought I would give you an update.
So in the end, I emailed my original migration agent and immigration. Immigration responded quicker, requesting copies of both job descriptions and my visa notification grant. I sent these through via email with a comparison of key duties in my current role and how they are addressed in maternity cover role. They have replied saying the jobs are primarily the same so can go ahead with the new role - super stoked!
Also, my migration agent replied this morning, saying the below:
"As you may be aware, subclass 457 visa holders are able to perform in the position that was approved in the associated nomination application. If your responsibilities will be altered, that needs to be approved by the Department before any changes are implemented. We would be happy to assist you with this notification and we estimate that the time required for this notification would be 4 hours (A$375 + GST x 4 hours)."
So a quick email to immigration directly, provided the confirmation I needed, and they also confirmed notes had been updated on my case file whilst saving me $1500 + GST!!