NSW Premier Kristina Keneally has ordered an investigation into a potential cancer cluster in Singleton, following reports in The Herald and the ABC's Four Corners that five cases had been detected in families living close together in the area.

In the past year, the NSW Government has approved four expansions of coalmining facilities in the Singleton local government area.

Another eight expansions are awaiting approval under the State Government's controversial Part 3A planning laws, which bypass council approval.

The most recent approval took place just weeks ago, with the Ashton coalmine given the nod for an $8.5 million expansion that would lead to an increase in run-of-mine production from 5.2 million to 5.8 million tonnes a year.

In March last year Hunter Valley Operations complex at Jerry Plains was given approval for additional mining and permission to process up to 16 million tonnes of coal at its HVO-south complex.

Greens upper house MLC Lee Rhiannon said the health study should include the entire Hunter.

"Focusing on just one suspected cancer cluster is akin to a GP examining a person's finger when a person presents complaining of feeling ill," Ms Rhiannon said.

"The danger of the Premier relying on the Singleton cancer cluster study alone is that an inconclusive finding will result because of the small number of people involved."

Minister for the Hunter Jodi McKay said yesterday she had never ruled out a health study for Singleton, but she believed it would be difficult to do a "statistically meaningful" comparative study with other areas because of the size of Singleton.

The NSW Minerals Council said yesterday that it took concerns about the potential health effects of mining seriously, but that pinpointing the source of the cancers could be difficult.