Canberra parents paid $3.5 million to public schools last year for extra resources including art supplies, library books and sporting equipment.
An Education Directorate official said the funds were "not even the cherry on top" of the government's investment in schools and an Australian National University academic said voluntary contributions were not known to impact on students' results.
But, ANU Centre for Social Research and Methods deputy director Nicholas Biddle added, "of course having additional money is better than not".
"It has the potential to make the school a more enriching experience for those kids who go to schools where parents are able to make that contribution," he said.
"The extent to which that's affected by your background characteristics or the background characteristics of the kids obviously touches on the extent to which it's an equity issue."
Sixteen of Canberra's 87 public schools received more than $50,000 in voluntary and subject contributions in 2016.
Nineteen received less than $10,000, including 10 with less than $5000 and two nothing. Financial data provided to Fairfax Media did not include tax-deductible donations made to library or building funds.