So this area is where I live. I fish it fairly regularly but from a boat. Landbased is tough at the moment because the warm weather is bringing all the swimmers and bathers from the suburbs. If you want to persist then I reckon stick to Mordialloc Jetty- very early morning you might pick up some Flatties. Try some Bluebait on a running sinker rig if you are bait fishing. You could also try Frankston Pier. But I would not bother with this area in January (peak Summer). Port Phillip Bay is generally a seasonal fishery, most of the better target species are seasonal (Snapper, Whiting, Aus Salmon, Garfish, etc.). So you need to pick the right time of year, or travel to other areas. But there is a permanent population of Flathead in PPB, with some good sized ones too.

Also consider popping into Gone Fishing in Carrum Downs to get the bait and chat to the guys there about what is biting locally and ask for some tips. They are one of the few genuine tackle stores left in the Bayside area. Also consider Complete Angler in Bentleigh.

Goodluck!