"It had been one of the coldest winters we've had here, so the Thames was rather fresh to tell you the least - the ice had just gone off the top of it."
He and his friend had just passed the London Eye and had gone under Westminster Bridge when they heard a lot of splashing coming from the river.
"We had a look over the edge and saw a guy on his back, but he kept bobbing under the water and coming back up again."
Mr Bonner, an expatriate Aucklander, threw one of the plastic liferings which line the Thames to the man, who was dressed in jeans and shoes, but he didn't grab hold of it.
"So I ran down the river, took my shoes and socks off and jumped in. I grabbed the life-ring and swam over to him ... and off we went down the Thames together," Mr Bonner said.
The New Zealander floated down the "very cold" Thames with the man for 15 minutes before they were pulled out of the water by two coastguard boats.
Mr Bonner suffered mild hypothermia but the man he saved was in a more serious condition.
"I ended up being a lot better off than him because I wasn't in the water as long and I was the one kicking and swimming to keep us afloat.
"He was in quite a state when he got out - he was a lighter shade of blue."
Mr Bonner has not been told what happened to the man since he rescued him but believed he was still alive.
He understood the man jumped into the river but as soon as he reached him "he was scrambling to get out".
"I think he regretted that decision as soon as he hit that water."
Mr Bonner's wife of two years, Alana Perrin, had a shock of her own that night after his friend sent her a picture of her husband wrapped up in the back of an ambulance.
"She wasn't overly impressed," Mr Bonner said.
But he has a sneaky suspicion that she has forgiven him after he was told last month that he was to be honoured for his act of bravery.
"I was pretty surprised, to be honest ... I got an email from the Royal Humane Society guy who said he'd been told by the coastguard what happened - it was quite cool."
He will be presented with his bronze medal by Princess Alexandra at the Royal Humane Society's annual general meeting on May 23 in London.
Mr Bonner and his wife, also from Auckland, moved to London about 4 years ago.
The Royal Humane Society grants bronze, silver and gold medals to those who have put their own lives at risk to save or try to save someone else.
The society was founded in London in 1774 by two eminent medical men, William Hawes and Thomas Cogan, who wanted to promote techniques of resuscitation.
It became apparent that people were putting their own lives in danger rescuing others and awards were given in recognition of these acts of bravery.
This remains the purpose of the society today.
Since its foundation it has made more than 85,000 awards.
