A 26-year-old man who was the subject of a Netflix documentary could be free from prison within 90 days.

Late last Friday a federal judge in the United States overturned the conviction of 26-year-old Brendan Dassey for his part in the murder of a woman called Teresa Hallbach in 2005.

Dassey's conviction 10 years ago proved controversial, not the least after it was featured on the Netflix documentary series "Making A Murderer".

He confessed to helping his uncle, Steven Avery, carry out the rape and murder of Ms Halbach in 2005 - a charge that Avery denies.

Dassey was arrested when he was 16, but his attorneys said his confession was coerced.

Federal judge William Duffin ruled on Friday that investigators had made false promises to Dassey that he "had nothing to worry about".

His ruling said the "repeated false promises, when considered in conjunction with all relevant factors, most especially Dassey's age, intellectual deficits, and the absence of a supportive adult, rendered Dassey's confession involuntary" under the US Constitution.

The US District Court in Milwaukee ordered that Dassey should be freed within 90 days, unless the case is appealed.

Journalist Emily Matesic, the Fox Valley bureau chief with WBAY News in Wisconsin, who also featured in the series, said the appeal was lodged before the documentary even aired.

"The appeal that was filed by Brendan Dassey's attorneys was filed before the Netflix series 'Making A Murderer' was released - so the judge based his decision on what was presented in the filing by Brendan Dassey's attorneys versus what was presented in the Netflix series," she told Newstalk Breakfast.

She added that the family are hopeful for his uncle, Stephen Avery, as well.