The NSW government has committed to combating Sydney's heat island effect by planting 5 million trees over the next 12 years, in a bid to offset the cracking pace of development.

NSW Premier Gladys Berejiklian said the government would spend $38.7 million over the next four years on its "Five Million Trees" initiative, as part of its plan to boost Sydney's existing tree canopy from 16 per cent to 40 per cent by 2030.

NSW Premier Gladys Berejiklian committed $37.5 million to planting five million trees across Sydney over the next 12 years. Louise Kennerley

While Ms Berejiklian trumpeted the policy as a first for any Australian state government, dozens of councils across the country have also implemented canopy policies and targets.

"No other state government that I know of anywhere in Australia has ever talked about tree canopy," she said.

"Wherever we can, we are going to hurry this process along and get more trees in the ground as soon as possible."

The initiative, first announced in November, comes as the government battles a perception of overdevelopment in the suburbs as it pursues ambitious housing supply targets.

Under the program the government will plant as many as 400,000 native trees each year until 2030 and will give away another 15,000 trees to people building homes in new land release areas in western Sydney.

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The government has come under fire for the felling of trees to make way for infrastructure projects, such as Sydney's eastern suburbs light rail, which resulted in scores of centuries-old Moreton Bay fig trees being cut down along Anzac Parade.

Ms Berejiklian conceded mature trees had been removed as part of the government's development agenda, but said the policy aimed to ensure "we are planting far more trees than we are removing".

"We appreciate nothing replaces a mature tree but we're setting ourselves up for the future by planting even more trees."

Every tree removed due to construction would be replaced by eight new trees, she said.

Using the example of Frenchs Forest, which has undergone significant development and land clearing due to the construction of a new hospital, the premier said the removal of trees was at times unavoidable.

"You can't expect us to stop building a hospital or a school or a rail line," Ms Berejiklian said.

"Not building that infrastructure is not an option."

Planning Minister Anthony Roberts said the government would work closely with councils and developers to ensure that tree planting was incorporated into the construction of new homes.

He said free trees would be given to people building new homes through the complying development approval process in western Sydney, so as to tackle the heat island effect which creates higher temperatures in the city's west than the harbourside suburbs.

Planning Minister Anthony Roberts said free trees would be given to people building new homes through the complying development approval process in western Sydney. Louise Kennerley

"That means reduction in temperatures in peak times around five degrees Celsius because we know trees are greater air conditioners. They will cool your home in summer and the deciduous trees keep it warm in winter."

The measure is incorporated into the Greenfield Housing Code, which requires a tree to be planted in the front and rear yard of each new home approved under complying development.

Fiona Morrison, the newly appointed Commissioner of Open Space and Parkland, will co-ordinate where many of the trees are planted as part of her role in overseeing the tree canopy policy.

The five million trees policy is part of the government's broader $290 million Open Spaces package, which includes $100 million to purchase land for public open space.