The nine-day working fortnight will only be feasible for workers if the Government helps cover their lost wages, say unions.
Helen Kelly, the president of the Council of Trade Unions, said the Jobs Summit's leading idea would become a nonsense if the Government only paid for training on the 10th day and put nothing towards wages.
"That doesn't necessarily mean total wages paid, but if the Government just pays for the training then most workers won't be able to afford to do it," Ms Kelly said.
"Why should they train for the employer's business without being paid? The scheme will become a nonsense."
Ms Kelly said she was concerned by comments made by Prime Minister John Key after Friday's Job Summit where he indicated the Government was more likely to pay for training than wage replacement.
On Friday, Mr Key made investigating the scheme a priority but ruled out the Government entirely paying the wages for the 10th day, saying: "I think it wouldn't be possible to fund it 100 per cent."
The cost to the Government of paying a one-day-a-fortnight wage subsidy has been put at $320 million a year.
Ms Kelly, who co-chaired the summit's employment session with Air New Zealand chief executive Rob Fyfe, said the recommendations were pretty clear that the Government had to put something towards wages.
Ms Kelly said she did not believe Mr Key's comments were the final call.
"Obviously they are trying not to make businesses leap in and think, 'yee-hah I can get one in every 10 paid for'."
Ms Kelly said the nine-day fortnight still had potential and unions wanted to make it work.
"There may be the possibility of the Government meeting some of the cost of wages, of businesses meeting some of the cost of wages, and even in some circumstances of workers meeting some of the cost of wages."
However, Ms Kelly said the idea that workers would "give up a day's pay to save a mate's job" was unrealistic.
The Labour MP for Mangere, Su'a William Sio, said the concern of his constituents - many of whom were in the type of firms that could be affected - was "who will pay workers for the days they give up?"
"If no one pays us, how will we meet our daily living expenses if we are forced to give up what would normally be a paid working day? If our incomes are reduced by one day will our fixed daily expenses such as rent, power, water, sewage, and phone lines be reduced as well?
"In Mangere, families aren't just concerned about job security, we are also concerned about income protection."
Ms Kelly said the scheme needed eligibility criteria, with firms being big enough to be able to run efficient training.
She said it needed to be flexible, so rather than shutting down for every 10th day, large operations could fit the training in at suitable times. She said it was good that employees could keep their jobs and improve their skills.