The woman went on to attempt to self-harm twice. Following both attempts, a number of officers applied significant physical force to restrain the woman.
The woman was then placed in a restraint chair.
The authority said officers should have placed the woman in a tear-resistant gown following her attempt to self-harm and should not have left her sitting in the restraint chair with no trousers on.
The woman was taken to hospital where she was found to have a fractured wrist, however, the authority was unable to determine exactly what caused this.
Authority chairman, Judge Colin Doherty said: "Ms X was a vulnerable person who was in need of proper care.
"The failure to properly assess her and check police records, meant she was put at risk by having access to the means to harm herself which itself led to unnecessary uses of force."
Bay of Plenty District Commander Superintendent Andy McGregor said police acknowledged they should have done more to ensure the necessary checks and assessment took place when Ms X was taken into custody.
"This would have alerted officers to Ms X's history and ensured an appropriate plan was able to be put in place to keep Ms X safe during her time in police custody, likely removing the need to use force to prevent Ms X from self-harming.
"Police are committed to providing the highest level of care to people in our custody but in this instance we did not meet the standard required.
"The issues highlighted in the report have been raised with the staff involved and the need to adhere to policies in place has been reinforced."
McGregor said earlier this year police started a national custody programme to "deliver an effective, safe and transparent custodial service that all stakeholders can have trust and confidence in".
