Malcolm Turnbull's standing as the Coalition's great communicator has taken a second hit in two days, this time when he was pressed on the cost of the centrepiece of his "jobs and growth" budget.

The Prime Minister's interview with David Speers on Sky News was not a train wreck, but it went as close as you can to having one without an ambulance being called.

Having announced a 10-year company tax cut plan as the key to secure prosperity, Turnbull should have been primed to answer the obvious question: how much will it cost?

Speers posed the question no fewer than 14 times in the interview, but did not get one clear, concise answer, providing Labor with a potent attack line: that the Prime Minister had delivered a budget without revealing the cost of its centrepiece.

First, Turnbull had told Speers (correctly) that the Treasury had not identified the 10-year dollar cost "of that particular item".

Then he suggested the cost was in the budget papers and pointed Speers to the page in the budget documents outlining projections for budget repair over the next decade.

His point was that the projection of a budget in the black was evidence that the tax plan had been factored in, except that the page in question observes that "small changes to underlying assumptions around the economy or future policy can have large impacts on projections of fiscal aggregates".

"So what is the cost?" Speers persisted.

Finally, before Speers moved on, Turnbull said economist Chris Richardson's estimate of around $55 billion may, or may not, be correct. No wonder Labor was in Parliament within minutes mocking the government for "a particular level of incompetence".

It was no consolation to Turnbull that his Treasurer was just an unimpressive when 3AW's Neil Mitchell pressed him on the same issue, at one point telling the broadcaster: "Well, I'll let you look it up mate."

The previous day, Turnbull had been coaxed into giving Labor another easy attack line out of an aggressive interrogation from Melbourne ABC radio host, Jon Faine.

When Faine asserted current policies were creating intergenerational conflict by locking "the kids of your and my generation" out of the housing market, Turnbull shot back: "Well, are your kids locked out of the housing market Jon?"

Faine: Yes.

Turnbull: "Well, you should shell our for them, you should support them, a wealthy man like you."

Faine: "That's what they say."

"Is that really the Prime Minister's advice for young Australians struggling to buy their first home?" Bill Shorten asked at the first opportunity. "Have rich parents?"

Turnbull is at his best when he can calmly, methodically and rationally explain complex issues and, in Tuesday's budget, he has something of substance to sell that plays to his and the Coalition's strongest asset going into the election - economic management.

But he also has the capacity to show his disdain for aggressive or loaded questions that challenge his intentions or, even worse, his integrity, and this has the potential to cause damage during a marathon eight-week campaign.

If there is an upside to the lapses, it is that they have underscored this danger to Turnbull and those around him before the campaign proper gets under way.

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