Last week, Mr Macri ordered that files on Mr Nisman's death be made public.
A year on, it is still not clear whether Mr Nisman was murdered or killed himself.
The prosecutor was found dead hours before he was due to testify in Congress against former president Cristina Fernandez de Kirchner.
President Macri was speaking to Mr Nisman's teenage daughters on the eve of the first anniversary of their father's death.
Mr Nisman's eldest daughter, Iara, had written in a newspaper article that she believed that "the death of my father was designed to create fear in people."
She said "I hope that we can find the truth about my father."
Mr Nisman had been investigating Argentina's deadliest terrorist attack, the 1994 bombing of the Amia Jewish centre, for 10 years before he was found dead in his Buenos Aires apartment.
Eighty-five people were killed as the building collapsed.
Much of the evidence was subsequently lost or contaminated, either deliberately or through incompetence, and no-one has ever been convicted in connection with the bombing.
The state prosecutor had been preparing to deliver a 350-page report to Congress in which he suggested Ms Fernandez may have covered up the alleged involvement of senior Iranian officials in the bombing.
Ms Fernandez has always rejected the allegations, claiming Mr Nisman was fed misleading information by a rogue intelligence agent trying to discredit her government.
The case has gripped Argentina - not only the investigation itself, but the country's investigative judicial service and forensic services have been under public scrutiny.
In February 2015, hundreds of thousands of people took part in a march in the capital Buenos Aires to mark one month since Mr Nisman's death.
Argentinian President Mauricio Macri has said he will ensure justice is done in the case of the death of State Prosecutor Alberto Nisman.