Opposition finance spokesman Barnaby Joyce says Peter Costello's critique of the coalition's paid parental leave scheme is not correct.
He also denies the federal opposition is "bleeding" over the issue.
Mr Costello - Australia's longest serving treasurer - says Opposition Leader Tony Abbott is engaging in a policy "race to the bottom" with Labor.
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Mr Abbott wants to give parents six months leave on full pay up to $75,000 funded by a levy on the nation's top 3000 companies.
Writing in The Age newspaper, Mr Costello said: "Alarm should have sounded like an air-raid siren once Bob Brown and the Greens lauded the scheme."
But Senator Joyce says Mr Costello can be wrong just like he is sometimes.
"I'm a senior figure in the National Party but just because I say something doesn't mean it's gospel," he told ABC Television.
"Peter's entitled to make those comments but it doesn't necessarily mean that his comments are correct."
Senator Joyce denied the former treasurer's column was damaging.
"I don't think we are bleeding on this issue.
"The community realises, unfortunately, we're in a position where we're not having enough children to be able to support the requirements that we so overwhelming want.
"Taxpayers for the future have to be born now."
Senator Joyce reiterated the coalition's plan involved a temporary levy not a tax.
It would be scrapped "as we start to get on top of this massive Labor party debt".
Tanner 'thrilled'
Finance Minister Lindsay Tanner was thrilled by Mr Costello's comments.
"Peter Costello's emphatically confirmed the government's attack on Tony Abbott's paid parental leave scheme," he told ABC Television.
"It's a great big new tax on business that will flow through in prices to consumers."
Liberal frontbencher Ian Macfarlane said Mr Costello had given up his chance to be involved in policy making as a member of the opposition.
"I always appreciate comment from Peter Costello, but I would have preferred that he raised his concerns with us and gave us an alternative," he told reporters in Canberra.
"He declined and that's Peter's prerogative.
"The mums and dads of Australia will decide if Peter Costello or Tony Abbott are relevant in this debate."
Outspoken Liberal MP Wilson Tuckey was more concise.
"Buzz off, Pete," he said.
"When you leave this place, it's not a bad idea that you keep your trap shut."
Greens disgruntled
Australian Greens leader Bob Brown said Mr Costello did not have an alternative to a paid parental leave scheme.
"I seem to remember Peter Costello was the fellow who totally botched the baby bonus," he told reporters.
"So come on Peter, be a bit constructive. Tell us what we should be doing after your 11 years in which you couldn't come up with a parental leave scheme."
Mr Abbott's scheme at least had some gumption about it, Senator Brown said.
