The organisers of ethics classes in New South Wales public schools say students are being placed on waiting lists, because the demand far outstrips the number of volunteer teachers.

Ethics classes were introduced as an alternative for students who opt out of scripture.

But they are proving so popular, the provider Primary Ethics needs to recruit nearly seven times as many volunteers as it currently has to meet demand.

General Manager Teresa Russell says religious groups have a ready-made recruitment pool in churches, temples and mosques.

She says it is not as easy to find ethics teachers.

"We don't have another meeting place during the week so we need to recruit the general public to reach the general public costs money," she said.

The classes are funded solely by donations and Greens MP John Kaye says it is not a level playing field.

"While donations to the providers of scripture are tax-deductible, those to ethics providers are not," he said.

He is calling on the State Government to ask its federal counterpart to make donations tax-deductible.