Please remember that, at 1500', the southern half of the orbit is in G airspace, which is not under the control of EN Tower.
The clearance for the orbit pertains to that portion of C CTR, MCG north to roughly Bolte Bridge.
It is convenient, probably wise, to stay on Tower frequency, as they may direct traffic to you, and you will be able to assess traffic approaching and exiting the zone.
However, at any time, there could be transitting traffic in G airspace that will be on another frequency, or in some cases just off in la-la land.
John E
The Yarra CTAF is only up to 1000' and is poorly marked on the VTC. I don't think the fixed wingy's pay too much attention to it. We know you're lookin' out tho'.
CG