The union which led the charge against zero hour contracts used by fast food franchises is now targeting Subway.

After encouraging McDonald's, Pizza Hut, KFC and Burger King to scrap the practice, Unite Union has written to Subway and Domino's Pizza seeking assurances they will do the same.

Union national director Mike Treen said Domino's told him most of its workers were on zero hour contracts but pledged to work with the union.

Subway responded by telling the union it had no control over employment at its individually owned restaurants, Mr Treen said.

He suspected most Subway workers were on zero hour contracts.

"They're not indifferent to the ingredients they use in their products, and they shouldn't be indifferent to their employment agreements either. We don't think that's acceptable in the 21st century."

He said some companies would only act if forced to.

Subway had not on Sunday responded to RNZ calls for comment.

The committee considering government legislation, which aims to outlaw the contracts, wrapped up oral submissions last week.

But the Law Society last month criticised the Employment Standards Bill for being worded in such a way that it has the opposite effect.

It said that in none of the proposed sections was there a prohibition on 'zero hours' contracts and in fact such contracts were enabled because, as there may be no minimum hours of work agreed, the hours of work may be zero.