Folbigg is serving at least 25 years behind bars after being found guilty in 2005 of the murder of three of her babies and the manslaughter of a fourth.

Lawyers for Folbigg lodged a petition in 2015 casting doubt on some of the evidence that led to her conviction.

NSW Attorney-General Mark Speakman on Wednesday said he'd formed the view an inquiry into her convictions was "necessary to ensure public confidence in the administration of justice".

"The distress that today's decision will cause is something that has weighed on me heavily in formulating this recommendation," Mr Speakman said.

"Whatever view you take of this case, it is a tragedy beyond imagination that four beautiful children were lost."

"We did it," Folbigg's friend Tacey Chapman told Seven Network.

"It did take too long but its an outcome we're all very happy for and I'm sure Kath's going to be very relieved like the rest of us."

Mr Speakman said the decision was not based on any assessment of Folbigg's guilt.

"I have spoken with (the babies' father) Craig Folbigg to explain this immensely difficult decision. I am sorry for the renewed distress and pain he and his family will endure because of the inquiry."

Mr Folbigg's brother John said he was frustrated by the decision.

"We're going to live it again, it's not nice," he told Seven Network.

"As far as we're concerned 15 years ago we got our answer."

Former District Court chief judge Reginald Blanch will lead the inquiry.

The inquiry is expected to focus on the evidence or lack of evidence related to whether three or more deaths can occur in a family with no explanation, Mr Speakman said.

It's expected to take six to 12 months.

If Mr Blanch finds that there is a reasonable doubt as to Folbigg's guilt he may refer the matter to the Court of Criminal Appeal for further consideration.

Australian Associated Press