There are fresh concerns within the federal government over Prime Minister Tony Abbott’s proposed paid parental leave scheme.

The scheme — the prime minister’s signature election campaign promise — offers working mothers on salaries of up to $150,000 a year full pay for six months of leave.

It’s slated to begin in July 2015 and predicted to cost about $5.5 billion a year.

Labor says the policy isn’t sustainable, and some voices within the government are now sounding out their concerns about the scheme as well.

Coalition backbencher George Christensen is worried there isn’t enough assistance offered to mothers who do not work.

“I really think that we need to be ensuring that there is something in place that’s going to be helping stay-at-home mothers,” he told ABC radio on Friday.

Liberal senator Ian Macdonald thinks the scheme will be “modified”, while his Nationals colleague John Williams says he’ll have a problem with the policy if the unemployment rate isn’t improved.

Mr Abbott has described the scheme as a “mighty social and economic advance” for women and families and has vowed to stick by it.