The trawlers get gurnard off that coast sounds like they’ve dumped the waste. Stingrays are bottom feeding scavengers I’ve caught a lot over the years. Squid is a great bait for them if you can get nice whole ones, otherwise kahawai and mullet work well. Sometimes I’ll use a whole small mullet or kahawai and cut a fillet off and put it back on flesh side up with bait cotton and can also wrap a bit of squid around them with bait cotton. They will also take small cut baits on ledger rigs. If I’m using whole fish I generally use a strayline type rig with a sinker at the top which leaves it on the bottom. I’ve hooked them at Bluff Wharf, Timaru Wharf, Lyttelton, Brighton Pier and Kaikoura. Most of my ones have been off the Pier, one night I hooked and beached 3 one after the other before I had to pack up for work the next day.
Got a few pics of various rays caught by me and a few other guys over the years mostly small and the two big ones I’ve hooked in the last year I’ve lost.
As for crabs they do eat them but I’ve never hooked one on crab and I know people who have fished with crab baits for years going after rig and never hooked a ray.