FACEOFF: The blackened out image of a man’s face representing the killer heart disease has been replaced with the face of Prime Minister John Key.

A Heart Foundation billboard depicting 'NZ's biggest killer' has been defaced with a sketch of Prime Minister John Key.

The billboard aimed at raising awareness of heart disease as "New Zealand's biggest killer" will now have to be taken down because the foundation can't afford to replace it.

It was put up on Commerce St, in the Hamilton suburb of Frankton early last month, and yesterday a member of the public told the foundation the image of a man representing the disease as a "serial killer" had been blackened out and replaced with a sketch of Key's face.

Heart Foundation head of marketing Vanessa Winning said employees and volunteers were upset the billboard has been used for political protest.

"It's really disappointing that a charity is being targeted to score political points," she said.

"I find it unthinkable that someone would vandalise our billboard, and consequently mock a seriously crippling disease, just to make a political statement."

It is believed the vandalism occurred sometime during the weekend and had happened only in Hamilton.

"We haven't had any other reports of vandalism," Winning said.

Hamilton West National MP Tim Macindoe's office has also been informed of the stunt.

The billboard is one of several around the country for the charity's nationwide annual appeal to raise awareness and funds towards fighting heart disease.

"Heart disease devastates thousands of families and communities in New Zealand every year," Winning said.

"This is an incredibly immature and callous act.

"People are diagnosed with heart disease every day and have to live with the consequences. To think that such a personal and destructive topic is being used to score points is very sad."

Whoever defaced the billboard went to " a lot of trouble" to measure and produce the image of Key, she said.

Winning said heart disease in New Zealand claimed more than one life every 90 minutes and many of those deaths were premature and preventable.

"We work hard to stop the prevalence of heart disease in New Zealand and an act of vandalism like this is very discouraging," she said.

"We don't have the money to waste on fixing vandalism of this nature. To rectify it saps precious money needed for our vital health programmes and research.

"Instead, we will have to take it down, as we can't afford to replace it."

The prime minister's office was disappointed to hear of the defacing.

"It's extremely disappointing for the Heart Foundation as this level of vandalism not only targets an important charitable organisation to make a protest point, it also leaves them with a hefty damage bill," spokeswoman Sia Aston said.