The Australian Coal Association released a report last week predicting mine closures and thousands of job losses if fugitive - or escaping - emissions of greenhouse gases were included in the proposed federal carbon tax.

The association's report has been criticised for exaggerating the impact but independent research shows a carbon price of $20 a tonne would add about $2 a tonne to the production cost of coal selling for $120 a tonne.

Lake Macquarie underground mines, especially the former Powercoal mines operated by Thai company Centennial Coal, were identified by the NSW Minerals Council as the most affected.

But inquiries by the Newcastle Herald have identified another seven underground mines that would be likely to incur the tax penalty, based on figures lodged with development applications and environmental registers.

They are Austar, Ashton, Blakefield, Dartbrook, Narrabri, Ravensworth and West Wallsend undergrounds.

While arguments prevail about the degree of "gassiness" - former mineworkers' union district president Mick Watson said last night the Hunter was nowhere near as "gassy" as the Illawarra - coal companies say fugitive emissions remain a problem in general.

While the amount of methane and carbon dioxide in coal seams varies according to the geology, it is generally the case that the deeper the seam the greater the concentrations of gas.

The federal government says fugitive emissions from coalmining account for between 5 per cent and 7 per cent of Australia's total greenhouse gas emissions, with coalmining "the fastest growing" of all fugitive emissions.

A recent conference paper by Centennial Coal said NSW underground mines produced about 10 million tonnes a year of carbon dioxide equivalent - or more than half the national total of 19 million tonnes from fugitive coal emissions, with Central Coast mine Mandalong accounting for 650,000 tonnes a year.