Naomi Watts says her baby is helping her lose weight.[Agencies]

Australian actress Naomi Watts has revealed the trick to losing weight - breast feeding.

The advice is not much help to women who aren't pregnant or men who need to lose the unwanted roll or two around their bellies, but new mum Watts says it works for her.

Watts gave birth to her second son, Samuel Kai, just a month ago.

On Monday in New York for the premiere of boyfriend Liev Schreiber's new film, Defiance, the actress showed off a svelte figure.

"I'm breast feeding and he's sucking it all out of me, it seems," Watts, 40, told People magazine on the red carpet when asked how she got back in shape so quickly.

"And when the baby comes out, it's a lot of weight right there."

Watts and Schreiber welcomed their first son, Alexander Pete, into the world in July 2007.