Can my family challenge a capacity assessment that declared my father incapable of managing property?

Yes, a finding of incapacity from a capacity assessment isn't necessarily the final word, though how you challenge it depends on what's actually happened as a result. If the assessment triggered a statutory guardianship of property through the Public Guardian and Trustee, there are mechanisms to have that reviewed, including seeking a further assessment or bringing the matter before the court, and a family member can apply to replace the Public Guardian and Trustee as guardian once incapacity has been established, or challenge the finding itself with proper evidence.

If the assessment is being relied on as part of a separate court application for guardianship, the assessment becomes evidence the court weighs alongside everything else, and your family can challenge its conclusions directly in that proceeding — for example, by obtaining a second opinion, questioning the assessor's methodology, or presenting other evidence about your father's actual functioning. Because capacity assessments carry real legal weight and challenging one effectively usually requires medical and legal evidence working together, get advice from a lawyer experienced in capacity and guardianship matters promptly, since delay can affect what options remain available.