I think its important to have a balanced property portfolio ie different property types, even different locations. So I suggest close to the city to balance the Wantirna South property but then again I am a proponent of being close to the CBD. You don't say what property type is your other IP. If its a flat (although I doubt it given the location), then disregard my advice below.
Unrenovated older style 1 bedroom (or maybe 2 bd depending on area) strata titled flat/apartment in inner city (up to 8-10km's from CBD - just draw a line north, east west, or south east it doesn't really matter to be brutally honest) with laundry facilities, preferably no lifts, no more than 12 but preferably 6-8 flats in the same block, car park on title, minimum 35 sqm in size, separate kitchen and common areas in good condition. Walking distance to train/tram. No flat roofs if you are on the top floor (hot in summer), so I would avoid top floor, as well as as real hassle for tenants if bringing their furntiture.
Will always be rented, low cost to update, maintain and owner's corp should be reasonable. Recent price escalations may make them negativey geared however even at 80% LVR.
Areas: East/SE - SYarra, Prahran, Hawthorn, Richmond, Windsor, Elwood
North: Carlton, Brunswick, Kensington, Flemington, Ascot Vale, Moonee Ponds, Essendon
West: Seddon, Newport, Altona North, Yarraville, Kingsville, West Footscray