Three-year-old Cheryl Grimmer went missing from a shower block at a New South Wales beach in 1970.
The Goodyear family gave a statement in 1970 and police, who believe they can still help with inquiries, launched an appeal to trace them.
The Nottinghamshire family, who said they were "surprised" officers had not found them, are contacting detectives.
Cheryl, originally from Bristol, disappeared from Fairy Meadow beach in Wollongong, a city south of Sydney, on 12 January 1970.
Despite a massive search, police failed to make a breakthrough and the case has remained one of Australia's longest-running mysteries.
Detectives doubt Cheryl's body will ever be found.
However, after the case was re-examined last year, a new clue emerged that led to the arrest of a 63-year-old man from Melbourne.
He has now been charged with Cheryl's abduction and murder.
New South Wales Police said they had been making inquiries with authorities in Britain to find Peter Goodyear, then aged 37, his wife Mavis and daughters Karen, aged six, and Janette, aged five, who were living at the Fairy Meadow Commonwealth Hostel at the time.
Police said attempts to find the Goodyears had "not been successful".
However, the BBC - which was inundated with responses to the appeal that was launched on Thursday - has since found the family in a Nottinghamshire village.
Mavis Goodyear, whose husband has since died, did not want to be interviewed, but told the BBC she had contacted the police concerning the case and felt "surprised" officers had not found them.
She said the family was unhappy with the publicity and felt police should have made more attempts to contact them before making an appeal in the media.
In response, a New South Wales Police spokesman confirmed they now had the details for the Goodyear family, and said they had tried to locate the family earlier "through the regular policing channels".
"Through the public appeal, we have successfully located the family and will be discussing the case with them when appropriate," the spokesman added.
A UK family sought by Australian police investigating a toddler's murder nearly 50 years ago have been traced.