I agree with Russki.
On the NSW Regional Development Northern Inland website, they clearly mentioned about the letter as below:
I think better to talk with your inviting state for a release letter if possible to minimise the potential issues
But again, the such requirments may differ from state to state
A “Release Letter”, releasing a 489 visa recipient from their obligation to this region may not be granted without the visa holder showing that they have taken serious steps to relocate to or remain in this region, through seeking employment. Such a letter can provide peace-of-mind when a skilled migrant needs to broaden their job search beyond their nominating region.
RDA Northern Inland has been informed that 489 visa holders do not need a "Release Letter" for their permanent 887 visa application.
Where someone has been advised by the Department of Home Affairs to obtain a release letter please send an email request to firstname.lastname@example.org with the following:
your current address
a copy of your visa grant notice from the Department of Home Affairs attached
an explanation as to why you are requesting the release letter