Nine Greenpeace activists have exposed holes in security at New Zealand's biggest thermal power station.

Protesters at the Huntly Power Station cut a perimeter fence at 6am and climbed chimneys planning to unfurl a 45-metre banner saying "climate changes start here".

It also had an arrow pointing to the station.

The protest was timed to highlight a new Greenpeace report on climate change to be launched in Wellington this afternoon. The organisation opposes the burning of coal because of carbon emissions.

The station, run by Genesis Energy, can generate 1000MW of electricity from coal and gas and is the only one in the country to use both fuels.

Genesis Energy this morning said the protesters had not caused any disruption to power production.

Police were at the site this morning but said it was too dangerous to make an arrest.

Of the security breach, spokesman Richard Gordon said Genesis Energy could never guarantee security of supply. He said the company was satisfied with its security and would not be reviewing it.

The power station employs security guards, uses security cameras and the 60ha site is fenced. Protesters damaged cameras and the fence when they broke in.

"If someone really wants to get in they will, without putting minefields, machines guns and turning Huntly into Fort Knox," Mr Gordon said.

Greenpeace spokeswoman Bunny McDiarmid said more than 15 members of Greenpeace entered the power station.

Ms McDiarmid said the aim was not to interfere with electrity generation but to highlight the toxic emissions caused by burning coal to generate power.

"Coal is the dirtiest of all fossil fuels. Huntly is probably one of our biggest sources of carbon dioxide pollutants in New Zealand," she said.

Ms McDiarmid said six people climbed the 150-metre high chimney and three went into the area of a 20-metre high coal conveyor belt. She said the protesters were expert climbers and had safety equipment.

Genesis Energy didn't send a rescue team to bring them down because they feared for staff safety, Mr Gordon said.

He said it wasn't Genesis Energy's responsibilty to rescue protesters.

Another part of the protest, to return coal from the site to Solid Energy's Rotowaro open cast mine, failed.

Mr Gordon said Genesis Energy was aware of the carbon emissions created by the burning of fossil fuels and said their new ep3 gas- fired station would be running in the next few months.

He said the Energy Efficiency Enhancement Project would save up to a million tons of carbon emissions a year.

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