I would think as soon as the land in the new estate sells out, which happens fairly quickly with the inner-city developments, any negative impact on CG for the surrounding properties (by there being vacant land so close by) goes away. You have to take into account that land + build is likely to exceed the cost of existing houses in the are as well, so CG impact could be then further reduced. However, buying on the border of new developments, the newer houses might end up more desirable to buyers than those just bordering it when sold as land + house. For example, if there's a new house 500m away from an old one (unless its a period/class home), there might be less interest in the old property. However, is the old house is significantly closer to public transport (or some other desirable amenity), then the tables could turn. It really depends on the circumstance I think.
Edgewater Estate sold out long ago, and I actually think that has become the priciest part of Maribyrnong now. The section west of Gordon Street, which would be the surrounding area to the estate, is still $$$ and I think has benefitted from that small shopping centre that was built on Gordon St.
What are people's view on the Valley Lake estate? Keilor East is coming up fast and this estate is now close to being sold out. Properties already built are fetching big bucks even with vacant land, it can only get better once all the land is gone? The only negative I see is the lack of immediate public transport, but apparently they're negotiating for one of the bus lines to come through, linking to the Niddrie tram or Essendon station. The estate isn't necessarily in public transport siberia (<5 minute drive to tram stop) but at least a bus stop in the estate is desirable to have. City access by car is fantastic.