Bertros, I think Dru was being genuine there mate, at least that's how I read it

Basswhisperer, in the summer you can chase pelagics in the shelter of the harbour but at this time of year you stand a much better chance around the heads / cliffs and inshore reefs such as long reef, the colours, etc just as others have said. If you've not done much offshore work before then I'd build up to it.
On the safety aspect, North Head, just like anywhere offshore, not only is it dependent on swell direction and size (and period!!) but also importantly the wind situation. Personally I'm far more wary of the wind than the swell (within sensible limits) when heading offshore. It's hard to say "anything under X metres will be fine" becuase it's not true. The important thing is what YOU are comfortable with and turn back if you're not. Personally I find North/Middle head far more "bumpy" than when heading offshore at places like Long Reef due to the reflection and refaction of the waves off the cliffs. But the great thing about launching at little manly as Paulb suggests is that you slowly progress into the swell conditions as you go around quarantine point and head out - most days you won't suddenly be in the thick of it so you can see how comfortable conditions are before continuing on and turn back if not.
Another safety consideration is "in how" you fish for pelagics. In general they are all strong and fast so be prepared for that. Leash your rods for starters, but also make sure your rod rests are secure and drags set appropriately - if a sizeable king grabs you're lure and your drag is locked up with a rod behind your seat or not easily/quickly accessible you could be in all sorts of mess very quickly. Trolling is a popular method and many people will have two lines out at a time but this can also be a danger. Pelagics are usually in schools and its all too common to double hookup. It sounds great but it actually takes some thought and practice to control the situation and not to end up tangled or worse. Even a rat king has the power to tip you if your drag is locked and you're not controlling the rod. Even worse for hobies is when one of them finds its way under the yak and tangles in your pedals! That happens frequently with Salmon that like to come up to the surface quickly after hookup. So for me I only troll two lines when I know one of them is a low-ish breaking strain and with the drag set a little lighter than normal - I then ALWAYS take the heavy rod first on a double hookup so I can let the fish run easily on the other and snap my way out of a tangle if one occurs.
Paul