A. juliana is particulary abundant on the walls of man-made ocean swimming pools in September. A. sydneyensis can also be quite common. I've looked at the marine lab web sites you suggested and I will also be contacting Drs. Willan and Steinberg for more info. Both the localities I mention below are the sites of marine research stations.
Numbers do go up and down from year to year, but there is never none.
The student could collect Dolabella auricularia on Heron Reef too. I'm a PhD student in zoology at the University of Washington working on defence in sea hares, particularly the ink and opaline systems of Aplysia . I'm looking for a site in Australia or New Zealand to do some field work on sea hares, particularly Aplysia species. I have contacted Richard Willan who studied the ecology of Sea Hares at Leigh Marine Laboratory , in New Zealand some years ago and you will find his reply above. If you can change the times you are likely to visit then Leigh, just north of Auckland would be a worthwhile place to work.