As soon as she heard the word "army" however, Mrs Rivers said, "I just shut down".
The officer on the other end of the phone said there had been an "incident in Afghanistan".
"I think I got out, 'Is he alive?'.
"He said yes, and I'll be there in half-an-hour."
Her son, Sergeant David Duncan, 36, had joined the army straight from school, having served in the territorials as a 16-year-old.
He had served in Bosnia and East Timor and before his deployment to Afghanistan had spent two years training others to go there.
He was on a shaky wharf in Lyttelton when the fatal Christchurch earthquake hit in 2011 and undertook duties in the city that included body recovery.
Mrs Rivers was therefore used to her son being in the danger zone, but nothing could have prepared her for that terrible phone call and the period of waiting that followed.
The officer arrived in about 45 minutes, having driven from Linton Army Camp near Palmerston North.
Unfortunately, "he didn't really know what had happened" - only that there had been an incident, that two New Zealand soldiers were dead and others, including David, had been injured.
Then followed "a really horrendous day which I spent basically howly-bagging".
Phone calls came throughout the day, whether or not there was news, as the army did its best to find out David's condition.
"The first reports are like Chinese whispers; they have to go through so many people," Mrs Rivers said.
"The first call said serious, the second call said very serious; I was going out of the room and crying."
The army officer was a friend of David's and on his first assignment to bring news of an injury to a soldier's family.
He was later replaced by "Ox", another friend of David's who was "absolutely brilliant", Mrs Rivers said.
"I can't praise him highly enough".
More encouraging news followed and Mrs Rivers learned that although her son had been seriously injured in both the thigh and in the arm, but was on his way to recovery.
Finally, late on Sunday night, the family was sitting by the TV when the call everyone had been waiting for came through _ from David himself.
``I just leapt to my feet and said, `It's David', and ran out of the room so I could hear him.
``My daughter and my husband followed me.''
The relief was something Mrs Rivers said she ``can't even begin to describe''.
``I knew he was alive but for a mother, you have to hear his voice.''
A few days later Ox and the army arranged to have Mrs Rivers and her husband fly to the United States to visit David and to accompany him home.
The army ``treated us like VIPs'' throughout the process, including some fast-tracking of passports, and priority treatment at airports in Samoa, Hawaii, San Francisco, and Washington, where at last they met up with David and began the journey home.
David's injuries happened during an ambush in which two soldiers were killed.
Mrs Rivers said David had got out of the armoured vehicle to help an injured comrade, who also survived.
``He was shot because he got out to help; he would have been safe in his turret on the cannon,'' Mrs Rivers said.
The group was under enemy fire even as they waited to be airlifted to safety.
A bullet had hit David in the right leg, entering through the inner thigh and breaking the femur clean in half, but missing the leg's major tendon and blood vessels. It took out a large chunk of flesh on the outer side of the leg as it exited.
Shrapnel had also lodged in his upper left arm.
David's first emergency operation was in Afghanistan, and then in Germany he had surgery to install a titanium rod, which is due to be removed later.
After some time behind a desk, David is looking forward to greater mobility and another overseas deployment.
Mrs Rivers said she is ``not happy'' but acknowledges, ``I don't think any mother can choose what their child decides to do.
``All you can do is support them in what they do decide to do.''
There is one bright side to the drama, however.
``I am really grateful that we had him home for Christmas.''
