Of 386 jobs summer jobs listed as available, about 150 were in a category including farm work, agriculture, fishing and forestry.
Manning said students were "incredibly reliant" on the work to pay their bills next year.
University of Otago student magazine Critic reported that 79 per cent of students responding to a recent online survey said work was harder to find.
Only 32.7 per cent had found a job, and 50 per cent had been unsuccessful so far.
Students were seeking a wide range of jobs: professional internships (26.6 per cent), retail (23.4 per cent), hospitality (13.8 per cent), and fruit picking (7.4 per cent).
Manning said students were Dunedin's "lifeblood", and any lack of summer work would also harm the city's economy.
SJS has reported a recent big surge in Otago job hunting: there were 12,083 registrations for work in the financial year to July 1, more than double the previous year's 5241.
Much farm and orchard work offered online by the service is at $18.90 an hour, the adult minimum wage, but other jobs start higher, some offering $24 an hour and a few job paying $40 and above.
SJS chief executive Suzanne Boyd said "post-Covid, the hospitality and retail industries are suffering", and students could struggle to find that kind of work."
Seasonal work, including fruit picking, was "attractive to students".
"They're most likely to apply for seasonal work if the pay is reasonable and the working environment is fair," she said.
