I'm also battling with this. In my case I have a heat pump installed and then later on a solar system as well, with a 450l geyser as storage. Never been happy with the performance, especially after the family shouts at me when they get a burst of cold water a few minutes into a shower.
Had a good look at the system today and it looks wrong on many levels. It looks like the cold water supply goes straight to the geyser, solar system and heat pump. The hot water return from the heat pump, as well as the hot water return from the solar panel, are Teed straight into the connection point in the middle of the geyser. Domestic hot water is taken from the top of the geyser. I don't see any NRVs in the system.
I'm not sure where even to start to debug and fix this setup. Any advice would be great.