Polding was openly gay, but police refused to disclose if he had been romantically linked with Pisano, who uses the nicknames Ree or Ricky, and is believed to be about 35.
Police said a post-mortem examination raised queries about how Polding had died and they had reason to believe Pisano could shed light on his last moments.
Sussex police spokeswoman Emily Bridges said the investigation so far had led police to believe that Polding's death was not linked to any hate crime as a result of his sexuality.
She added that any assault he may have suffered "was not as a result of a random attack".
It was highly unlikely Pisano had returned to either New Zealand or South Africa, but they had alerted both countries and all UK ports, airports and police stations.
"We know that he is from South Africa and he had travelled to London via New Zealand. At this stage there are very limited details known about him."
The head of Interpol South Africa, Brigadier Scott Naidoo, said an international wanted persons alert, known as a Red Notice, had not been issued for Pisano.