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In a report released today, the Independent Police Conduct Authority (IPCA) found the officer's use of force was unjustified, because she had other options.
The incident occurred on February 13, 2019.
A man in the custody of Dunedin police "appeared to self-harm", before resisting two police officers attempting to restrain him and kicking one of them, police said.
An officer referred to as Officer C used a Taser after the man challenged and moved towards her.
Southern police district commander Superintendent Paul Basham said Officer C felt threatened but police accepted the authority's finding that using a Taser was not the most appropriate option.
"Police officers are put in high-pressure situations every day where they are required to make quick decisions about their own safety, as well as the safety of those around them," he said.
Officer C should also not have been carrying a Taser in the custody area, he said.
"Police strive every day to provide the best possible service and support to our communities but it's important to recognise when we don't get things right.
"Police have now updated Taser policy to make it clear that staff must remove Tasers when entering custodial areas."