He said in countries where a structured law oversaw assisted euthanasia, there was greater assurance of people's wishes being respected, suffering was minimised and unethical practices were not possible because of the openness of the process.
Laws gave people reassurance "that if the worst befalls them, then they have the choice of asking for help" and they protected them from collusion.
In the Netherlands, there was no avalanche of assisted suicides and the basis of eligibility was "unbearable and untreatable suffering".
For some, palliative care could not relieve their pain, and many suffered from mental anguish because of the state they found themselves in. One patient described her state as being "locked in a scream".
People in Australia were taking their own lives "often in horrific circumstances" when diagnosed with a terminal illness.
During questions after Mr Denton's presentation, several people gave an emotional account of the death of a loved one.
Euthanasia-Free NZ executive officer Renee Joubert said she agreed with Mr Denton that official Belgian studies showed the system was working, but independent surveys with respondents given anonymity showed just 50 per cent of assisted suicides were reported.
Mr Denton said some doctors did not consider their action euthanasia and were very independent. "It is certainly a lot better [in Belgium] than a society like ours where things are happening and there is zero oversight whatsoever."
Ms Joubert said Mr Denton had offered a false dilemma of two extremes: change the law or people suffered, including "lonely violent suicides".
"We have a system already that regulates what doctors are doing - if there are issues with that then certainly it can be developed," she said.
Mr Denton interrupted her, saying: "These are not issues - these are people dying horrible deaths."
She said people often suffered from under-treated depression when terminally ill: "Could that be the real reason why people want to die?"
He said it was "an enormous assumption" to blame depression, "but if you have a law where this can be openly discussed then you have far more chance of dealing with these questions".
She said people on both sides cared about suffering "but changing the law is not necessarily the way to go about it".