The Tribunal heard Speirs, who has a doctorate in psychology, had implied to the Family Court in 2017 the man had sexually abused or would sexually abuse his daughter.
The opinion was given based on factors such as his willingness to change the girl's nappies, and his habit of showering with the door open.
The tribunal found Heron-Speirs did not have enough information to form this opinion, and had presented herself as an expert, while failing to provide evidence in an unbiased manner.
They found her actions amounted to professional misconduct, was unethical, and likely to discredit the psychology profession.
Professional Conduct Committee lawyer Anita Miller previously said the case involved "complex and difficult family circumstances".
The complainant was married to the child's grandmother and had what he described as an affair with his wife's 21-year-old daughter. The child was the result of that affair.
The man said the affair was consensual, but the child's mother said it was a one-off incident of rape. Police have investigated the claim and have not laid charges.
An affidavit from Heron-Speirs came several years later amid family court proceedings in which the man was seeking partial custody of the girl, who was then 4 years old.
The affidavit included concerns with the man's keenness to change the girl's nappy, saying it "provided him with an opportunity to molest her", and noted the ex-wife's concerns about the girl getting into bed with her father as it created an opportunity for him to "encourage her to touch his genitals".
She also referred to his ex's claim he repeatedly showered with the door open, labelling the behaviour as "abusive" and suggesting "improper intent".
Heron-Speirs said he had "shown a determination to breach safe sexual boundaries" and that his behaviours could have a "conditioning effect" on the girl.
The psychologist earlier made reports of concern to Oranga Tamariki and police, but was told the girl had not disclosed any abuse.
Heron-Speirs never met or spoke to the man, let alone assessed him or observed him with the girl, the tribunal heard.