It will depend on what you want to see and photograph, if you have a month go north and south to see the difference.
Sydney has brilliant shore and boat diving - Bare Island and Kurnell (both shore diving sites in Botany Bay) have weedy sea dragons and other great creatures.
South of Sydney is all temperate diving, great spots are Bass Point, Jervis Bay, Ulladulla, Batemans Bay, Narooma, Eden and Merimbula. I would recommend Narooma at that time of the year to dive Montague Island - hundreds of fur seals, also port jackson sharks, pelagic fish, stingrays, grey nurse sharks and the visibility is rarely less than 20m. (Note the PJ aggreate over winter, June-August, so are as common over summer)
North of Sydney, Port Stephens - best shore diving in Australia - great muck and beautiful sponge gardens packed with critters. Forster/Seal Rocks - great boat diving and lots of grey nurse sharks, especially at that time of the year.
South West Rocks is always brilliant and the start of the subtropics, more coral reef fish and corals. Coffs Harbour has wonderful boat diving around the Solitary Islands and if you make it all the way to the border Byron Bay is spectacular - lots of wobbegongs, reef fish, pelagic fish and at that time leopard sharks and manta rays.
Hope this helps.