Whitehaven Coal is continuing its expansion, launching a $172 million offer for NSW coal explorer Coalworks.

Whitehaven took a 17 per cent share in Coalworks when it bought Aston Resources for $720 million in April.

Now, Whitehaven has offered $1 for each remaining Coalworks share in an off-market takeover bid, well above the 85.5 cents the shares last traded at.

Coalworks has three projects currently in production in NSW - the Vickery South project in the Gunnedah Basin, the Ferndale project in the Hunter Valley, and the Oaklands project in the state's southwest.

Whitehaven said it believed those projects could be developed more efficiently if done so in combination with Whitehaven's existing assets.

The company has five open cut mines in the Gunnedah Basin.

"The acquisition of Coalworks represents a logical next step for Whitehaven which will allow Whitehaven to consolidate its development and exploration portfolio," managing director Tony Haggarty said in a statement.