That was not uncommon, Mr Burdett said.
"It's common for people to ask for somebody - usually it's a family member. In this case, he obviously knows him because he's asked for the person ...
"Negotiating and crisis intervention is always about rapport-building - always. You can't get away from it. You have to become a friend with that person, to a degree."
Up to 10 officers make up a negotiating team, Mr Burdett said. A main and secondary negotiator would have been dealing with the gunman personally.
It was likely Mr Morehu would have been asked to speak to Warren over the phone - but would not have been allowed to meet him face to face, for the same reason Warren's whanau were not allowed through the cordon.
"They [besieged gunmen] can change in a dime. Some people just switch. It's a very volatile situation."
Mr Burdett said during such a stand-off, a person would go on an emotional rollercoaster. "They hit some self-reflection. Often they go down and have remorse and a lot of guilt. You can't stay angry for a long time. Then the opposite - you end up going down this big dip."