Community lobby group The Australian Coal Alliance said the comment was a "bombshell that has major implications for the Hunter".

The Northern Sydney Central Coast area director for public health Peter Lewis made the submission to the Department of Planning about a South Korean company's plan for the Wallarah No 2 mine in the Wyong Shire.

"Any increased exposure to particulate pollution is associated with increased adverse health outcomes, even if the levels are below the current guidelines," Dr Lewis wrote.

Dr Lewis said increased particulate exposure could cause deaths, require hospital admission, and make children have more chest colds, night-time coughs and trips to the doctor.

Coal alliance spokesman Mike Campbell slammed the state government for not making the document public.

Mr Campbell said the government must review air pollution limits.

"It is a big step forward from a government official to say 'yes there are problems within the current guidelines'," Mr Campbell said.

Dr Lewis's submission said mines on the scale of the Wallarah 2 plan "must be considered in the context of the whole region, not as a stand-alone development".

Mr Campbell said many Hunter coalmines were similar in size to the Wallarah 2 plan.

He said he asked the department for a copy of Dr Lewis's submission but was told "we don't have a copy here, I'll get back to you".

"I had to get it off the author himself," Mr Campbell said.

A Department of Planning spokesman said that before July 1, 2010, major project submissions were not made public.

But the policy had been changed so submissions made after July 1 would be available on the department's website, he said.

Dr Lewis's submission was made on June 9.

"Issues raised in all submissions, including from the health service, will be carefully considered as part of the department's assessment of the [Wyong mine] proposal," the department spokesman said.