Kathleen Maier wipes away tears as she reads the last email message she received from son Judah.

"Just letting you know I'm eating honey-nut cheerios and thinking about you," it read.

"It's so Judah; just irrelevant," Maier laughs.

Maier was winding up five months of aid work in Ethiopia and planned to travel before returning to Christchurch.

Instead, she has returned home early to prepare for her only child's funeral.

Judah, 19, died after a head-on collision between his moped and a four-wheel-drive vehicle on Christchurch's southern motorway last Thursday - Maier's 50th birthday.

He had apparently become confused by the road layout and rode into the wrong lane.

Judah had spoken to his mother about an hour before his death, during a dinner in her honour with friends.

Maier said she had tried to Skype but the group was out. On a whim, she decided to spend the extra money and call their cellphone.

"It was the best decision I ever made."

She could not remember the details of her last conversation with her son, but it was "positive and fun".

It was not until the next day that she got the "absolutely horrible" news of his death. When her phone rang at 3am, her first thought was someone had got the time zones confused and was phoning to wish her happy birthday.

However, she could tell by her father's voice that something was wrong.

" ‘Kathleen', and there's this pause," Maier said.

"He said, ‘I've got terrible news'. My first thought was something's happened to my mum.

"He said, ‘Judah's been killed'. I just screamed.

"How can that happen? How can a kid who loved life, how can he be gone?"

She left Ethiopia on Saturday and arrived in Christchurch on Monday. A funeral is scheduled for tomorrow at the Spreydon Baptist Church.

Maier said her son, born in Bulgaria, was "a gift" from day one. "I always had a sense he was on loan. I just had the privilege of taking care of him."

Judah spent his childhood in Bulgaria, Canada, New Zealand, Albania and the United States, where he got the taste for honey-nut cheerios.

Maier said her son "lived exuberantly and loved exuberantly". Ever-playful, he would take friends to Christ Church Cathedral where Maier worked and joke with then-dean Peter Beck and other cathedral staff. She was also impressed by his maturity while she was away.

"He added so much to who I was. He kept me lighter than I would have been without him," she said. "So many people were touched by him. That helps me. We tried to make moments count."

MOTORWAY CLEARED OF BLAME IN DEATH

The NZ Transport Agency (NZTA) says "everything was right" on the section of the southern motorway where a moped rider died in a head-on crash last week.

The agency started an investigation after Judah Maier, 19, collided with a four-wheel-drive vehicle on State Highway 73 near the Wrights Rd overbridge about 8pm on Thursday last week.

Police said it appeared Maier, heading east, was confused by roadworks, possibly thinking it was two lanes instead of one lane both ways, and rode into the wrong lane before the crash.

He died at the scene. NZTA highway operations manager Peter Connors said the section of road where the crash happened was a work site, but the road had been unchanged.

Two additional lanes were under construction adjacent to the overbridge where the crash happened and the road layout would be changed eventually, he said.

Connors said the NZTA had checked the crash site and found the only thing amiss was a street light being out.

The transport agency investigation will end this week.

- © Fairfax NZ News