The Lemons tend to be wherever you find them in HB. Sometimes you get them on every beach, other times they're hard to find.
Any of the beaches north of the port produce lemons now and again, but I've also caught them at Awatoto, Haumoana, Te Awanga, and Ocean Beach...
If crabs are taking your baits, it generally means there are no lemons around.
If the crab activity suddenly stops, time to throw out a prawn or crab bait