Hadley says he was initially skeptical about the call, so he spoke to a young man off air.
"I spoke with a young man who was having instructions barked at him by a person … speaking in English but with an Arabic accent," he said. "That person told him that he wanted me to convey that he wanted to speak to our Prime Minister Tony Abbott, that he wanted an ISIL flag conveyed to the café … there were other demands, they were quite strange."
The radio host gave police the mobile phone number, who confirmed it was a hostage they had spoken with earlier.
Police asked him not to broadcast the information, a request he complied with even as more calls came in during the unfolding crisis. A negotiator was sent to the studio to handle the calls while Hadley stayed on air.
"I've been doing this for 30 years, and I've never been in this situation before," he said.
"I'm not a negotiator, I'm a radio host."
Click the video above to watch the full interview with the Sydney radio host.