Police spent $36.7 million dealing with mental health callouts in the 2015-16 year but the Labour Party says that is just the tip of the iceberg.
Labour obtained the figures under the Official Information Act which show police now estimate they spend 600 hours a week on mental health incidents.
Labour's leader Andrew Little said the true cost was likely to be much higher.
According to the figures in 2015-16 the police attended 12,398 mental health incidents and 16,179 attempted or threatened suicides. It took them over an average 2.5 hours to resolve a mental health callout and 3.5 hours to deal with attempted or threatened suicide calls.
A 2010-2011 study by police found that half of those who committed suicide in that period had had contact with the police in the year leading up to their death, either as an offender or victim.
The figures for the 2015-16 year recorded a 1 percent increase on the year before for mental health callouts and a 10 percent increase for attempted and threatened suicides.
Mr Little said the document confirmed a new approach to mental health was needed.
"These statistics from the police's own database show that these incidents are increasing and reveal the growing stress on our mental health sector. The $36.7 million is only the tip of the iceberg as it only takes into account the cost of police callouts that are mental health-related where no crime occurs."
He said the present service was over-stretched, fragmented and with little co-ordination in the care of people. Labour would set up new mental health teams to offer free treatment to people suffering mental health problems.