Look for structures that have kelp growing around them or nearby.
Bridge pylons, pontoons, jettys wharfs or walls.
I haven't tried fly for them, but you need to berley them up.
We do get them year round in the harbour, with the exception of August.
I think it just seems like they are a winter species, when other fish come on in the warmer months not many folks still target blackfish.
They chase kingies etc.
As for weed aha..... A man could get himself in serious trouble.
There is heaps of cabbage weed on the ocean rocks at low tide.
Check rock pools, and ocean baths for the string weed.
The string seems better bait in the harbour, but the cabbage also works.
I usually grab a bit of both, berley with both, start fishing with string and have cabbage as a backup.
Ryder