Mr French said the note-giving method was "fairly common".
He said he could not comment on how much money Mr Herkt had walked away with.
Officers have been investigating since the first offence, but previous leads on addresses in Papakura, Papatoetoe and Auckland city have been unfruitful, he said.
"We've been talking to any known associates that he has and we've identified any addresses that he's been at previously, we've been visiting those addresses but haven't located him to date."
Mr French warned the public to not approach Herkt, but to instead call police as there was a warrant for his arrest.
"Although no firearm has been sighted, in his notes he's threatened that he does have one, so we have to treat him as being dangerous and possibly armed."
He asked anyone with information about any of the incidents or the whereabouts of Ben Bosch Herkt to call Counties Manukau Police on 09 261 1300. To provide information anonymously, call Crimestoppers on 0800 555 111.