Cate Blanchett has taken a thinly veiled swipe at Australia's adoption laws, confirming she and her husband, Andrew Upton, adopted their baby daughter from the United States.

"I think it is… well I haven't been through it here. It was in the United States," she told Fairfax Media with a nonchalant shrug of her shoulders when asked for her thoughts about Australia's adoption laws and legislation.

Speaking on the blue carpet at the Australian premiere of her new film, Cinderella, the 45-year-old shared the news of her fourth child – a baby girl called Edith – for the first time.

"There's a lot of children out there that don't have the good fortune of our biological children so it's lovely to welcome a little girl. We're besotted."

Edith joins the couple's three other children, Dashiell, 13, Roman, 10, and Ignatius, 6.

The family are due to relocate permanently to the United States at the end of the year once Upton's term as the Sydney Theatre Company's artistic director comes to an end.

"We're all looking forward to spending more time together in one place. We spent nine years living in England and we loved it. But now we'd like to try America. They have terrific [opportunities in] television there and Cate has a very strong film career there, too, obviously," Upton said.

The Upton-Blanchett family follow in the footsteps of other high profile Australians, such as Hugh Jackman and Deborra-Lee Furness, who also adopted their two children through the American system.

Furness, an outspoken adoption campaigner, founded Adopt Change, a lobby group that is now working to change Australia's "anti-adoption" culture, a hangover from the horror of the stolen generations and forced adoption of the 1960s and '70s.

In 2013-14, 317 babies were adopted in Australia, a 76 per cent fall from 25 years ago.

In Australia, adoption is independently controlled by the states and territories. Prime Minister Tony Abbott has said the federal streamlining of laws is on the agenda for upcoming COAG meetings.