For the lake, strong NE winds are the worst, or a swell from E to NE. Even then, on the flood the bar is usually ok. Contacting the VMR for on site observations is sometimes productive, but some of the operators tend to be a bit stingy with info.
IF you are coming from the north, Port STephens, some 40 miles before the lake (IIRC) is essentially an all-weather port and has both marinas
and good anchorages
available. About 15 miles north of the lake is Newcastle harbour on the Hunter
river. This entrance, used by giant coal ships, is almost never a problem for yachts. Once inside one can choose to go to the marina or anchor
in the north arm of the river about a mile in from the entrance. Very good protection and holding, very industrial scenery... not pretty but safe and easy!
Hope that helps, mate... have a good trip. The lake is an interesting venue and should you tire of it, Broken Bay is not much further south and has lots to offer.