Sounds like you kinda need to harden up first of all 
But in all honesty, I'm in western syd as well and I wear a hoody under my leather jacket and that's about it. Depending on what I chose to put on that day I might have a long sleeved shirt under the hoody.
Leather in and of itself isn't particularly warming, I don't think. If you have a winter liner then it should be a bit warmer. My old jacket had one, current does not... but I can't say I've noticed a difference.
Biggest factors just from what gets cold on me would be gloves and pants... sort those out and leather + underneath a jumper/hoody etc should have you right.
But in all honesty, I'm in western syd as well and I wear a hoody under my leather jacket and that's about it. Depending on what I chose to put on that day I might have a long sleeved shirt under the hoody.
Leather in and of itself isn't particularly warming, I don't think. If you have a winter liner then it should be a bit warmer. My old jacket had one, current does not... but I can't say I've noticed a difference.
Biggest factors just from what gets cold on me would be gloves and pants... sort those out and leather + underneath a jumper/hoody etc should have you right.