Callaghan (33), a structural engineer, changed his plea at the High Court in Auckland, admitting he killed Ms Thomas on June 29 last year in Auckland.
Comm Marshall, who replaced Howard Broad as police commissioner eight months ago, was in Queenstown on the final stop of his southern district tour, meeting police officers in the South.
The White Ribbon campaign, which aims to end violence against women, had been the subject of "a lot of discussion", he said.
"Twenty-four death investigations this year involving family violence [are under way or have taken place].
"Ten involve the deaths of children.
"Our record is not proud here in this country and we have increased reporting, which is great.
"We want people to have the confidence to report," Comm Marshall said.
Police introduced family violence orders last year - providing officers with the ability to instantly remove an alleged offender from a home for up to five days.
In the southern district family violence incidents were fewer than elsewhere in New Zealand, however, "one is too many", Comm Marshall said.
"Everybody has got to be responsible."
Comm Marshall had almost 40 years' experience with the police, most recently on secondment from the New Zealand Police as the Commissioner of the Royal Solomon Islands Police and requested a three-year term, instead of the usual five, after which he planned to "grow a beard and drink some wine in Central Otago".
Overall he was thrilled with the southern district, saying "atta boy Queenstown" to Senior Sergeant John Fookes, southern district Commander Bob Burns and the management team.
"I've got to take my hat off to the management team ... I wish them all the best."
