LONDON - A mystery woman believed to be from Australia or New Zealand has become the fresh focus of the worldwide search for missing British toddler Madeleine McCann.

Private investigators hired by Madeleine's parents Gary and Kate McCann were alerted about the woman by a new witness who detailed a conversation he had with her in Barcelona just three days after their daughter vanished from a Portuguese holiday resort in 2007.

The investigators believe the woman could be of "potential significance" to their search for Madeleine.

They are due to release images of the woman, described in some media reports as a Victoria Beckham look-alike, and make an appeal for anyone who knows her to contact them at a news conference in London on Thursday.

"What this Australian is reported to have said (to the witness) is of enough significant interest to launch a hunt for her," a source close to the investigation told The Sun newspaper.

"We don't know where she is. She could still be in Spain, back in Australia, or anywhere else. But we need to find her quickly."

The man who told investigators about the conversation he and a friend had with the woman outside a Barcelona bar is British and only recently came forward with the information.

One of the men believed the woman had an Australian accent, while the other told investigators he believed she was either from Australia or New Zealand.

Madeleine was just four years old when she disappeared from her family's rented holiday apartment in Praia da Luz on May 3, 2007, while her parents dined with friends at a nearby restaurant.

Despite a massive search, Portuguese police wrapped up their investigation in 2008 after failing to find any trace of Madeleine.

Her parents hired their own investigators to continue the hunt in the belief that their daughter is still alive.

Despite the new link to the woman, Australian Federal Police (AFP) have not been asked to help search for her.

"We haven't received a request for any assistance," an AFP spokeswoman said.

"So at this stage we are not investigating any of the allegations made."

To mark the two-year anniversary of their daughter's disappearance, Kate and Gerry McCann made a fresh appeal in May on their website for help with their search.

"Whilst the fact that Madeleine is still missing remains our greatest anxiety, it is important to remember that the person who took Madeleine remains free," they said.

"There is no law enforcement agency actively looking for Madeleine or the person who took her.

"As such, Madeleine and possibly other children are dependent on good and moral everyday people to help them. We kindly ask you to share this responsibility with us."

- AAP