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The two accused are Marco Leiva Soda, 34, and his nephew Adrian Soda, 20, Los Andes reported.
The newspaper said both men, from San Martin, were arrested after police raids on neighbourhoods in the west city.
Security Minister Leonardo Comperatore said the murder weapon was not found and "we will continue with the investigation by the prosecutor Garay", Los Andes reported.
He said security camera footage was still being analysed, and police would have a video conference with a French friend of the victim who saw the killing.
Mr Heyward, 31, was killed on April 28 when walking with two other people near Malvinas Argentinas Stadium.
He was attacked by two men on a scooter, one of whom tried to snatch his bag but failed, and shot the New Zealand four times.
One bullet hit him in the neck and another in the abdomen.Mr Heyward, a physiotherapist, was born in New Zealand but had been living in Australia.