We've got a 6kw solar system, installed 5 years ago, no problems with it. Howver, its been installed facing North, which is great. Can i just add more panels and another inverter or do i need permission from anyone?

Since your system was installed the standard for approved panels and inverters has been updated. This means that if your solar system is modified, the entire system will have to be upgraded. At the very least you would need a new inverter, possibly new panels and isolation equipment too. If it was well installed and working well then the best bet is to leave it alone, but have a simple safety check - I recommend one every 5 years.

South-west facing arrays can make sense.  In the Sydney area they will produce about 75% the energy yield of a North facing panel.

The best solution is for a whole new 3kW system next to your existing one. You do need permission from your network to add more solar. If you are on single phase power, the only issue you may come up against is network approval for 9kW of solar on a single phase. But your installer will be across your local connection rules.