Council’s State of the Environment says pressures of population boom pose biggest danger.

The biggest threat to Auckland's environment continues to come from its own booming population, according to a comprehensive stocktake being presented to city councillors today.

Auckland Council's 2015 State of the Environment report - the first since 2009 - has presented a mixed bag of wins and woes when it comes to how the city's blue and green backyard is coping.

While there have been some regional-level improvements in air quality, and gains in protecting native species in places that are intensively managed, the continuing flow of sediments and contaminants from the city has kept marine and freshwater environments in slow decline.

These pressures all come back to the pace of Auckland's growth, and current projections suggest its population could hit 2.01 million by 2033 - an increase of more than 517,000 within the next two decades.

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The 200-page report found that this and the associated needs for transport and heating - particularly with more diesel vehicles pumping out particulate matter and nitrogen dioxide - could see positive air quality trends reversed.

While Auckland had areas of high biodiversity - specifically on well-managed islands and large tracts of remaining indigenous forest on the mainland - this could be slashed where intensive farming and urban growth began to eat into the countryside.

The report noted how the proposed Auckland Unitary Plan had already indicated that the urban area might expand over the next 30 years into the designated Future Urban Zone, bordering the rural-urban boundary.

This zone covered just over 10,800ha - an area that could accommodate 90,000 dwellings.

This scale of development was in step with a dramatic change in land use, which, since 1998, had seen the area of land in parcels of less than 4ha increase by 60 per cent.

Report co-author Dr Megan Carbines, a senior scientist with the council's research and evaluation unit, said Auckland's environment was facing the same population-driven pressures seen in other cities across the world.

"More people means more people on the roads, more land required, and more housing required," Dr Carbines said.

In his foreword to the report, Auckland Mayor Len Brown said the challenge now was to encourage communities to find ways to "move faster" on some of the city's most pressing environmental problems.

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"As Auckland grows, it must grow greener," he said.