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The New Zealand Herald understands Ms O'Brien's former partner, Matt Ruddell, was yesterday at his children's bedside in the Starship hospital in Auckland.
Mr Ruddell, originally from Northern Ireland but working in Tauranga as a chef, posted on Facebook that his daughter Faith, 3, who was initially thought to have suffered serious injuries, was awake yesterday morning.
"Faith is now awake head injury not as serious as first thought!!! But she's doin good," he said.
"Fee [son Phoenix] still in an induced coma ... undergoing traction to reset both his legs etc. I will keep u all posted xxx to my work, family. I love u guys."
Phoenix, 4, was yesterday in the paediatric intensive care unit and his sister was recovering in a ward.
Last night, friends of Ms O'Brien, 26, gathered at Mount Mellick Irish bar in Mt Maunganui to remember her.
Owner Clayton Mitchell, a Tauranga City councillor, said she was a regular at the bar and would be missed by all the staff.
"She was really a lovely, cheerful, friendly and seemingly always gregarious customer. I think it's hit the community really hard. She was just a really positive, upbeat person that was just there for everybody."
Mr Ruddell's workplace has set up a givealittle page to donate funds for him and his two children. Last night it had raised more than $2000.
On its Facebook page, the Phoenix restaurant in Tauranga said it was a heart-wrenching tragedy.
"Our thoughts and prayers go out to Tracey's family and friends on what is such a tragic ending to such a beautiful young lady at only 26 years."
The restaurant said it would give 10 per cent of its sales on Mother's Day for "little Phoenix and Faith".
Ms O'Brien's car crossed the centreline and collided with a people mover on State Highway 2 outside the Charlemagne Lodge wedding venue at Te Puna about noon on Saturday. She died at the scene despite the efforts of groom Jordan Mountfort and his groomsman. The two children were rushed to Tauranga Hospital before being flown to the Starship.
The two adults and four children in the other vehicle were taken to hospital with minor injuries.
Bay of Plenty police communications manager Kim Perks said it was too early to speculate what happened to cause the crash.
However, Mr Ruddell - who separated from Ms O'Brien last year - has posted on his Facebook page a theory.
"May this be a massive wake-up call to everyone - never txt and drive."
Ms Perks said police wouldn't rule out the possibility Ms O'Brien was texting before she crashed, "but equally we wouldn't rule it in either".
Later yesterday, Mr Ruddell posted about his former partner: "Taken to soon!!! Words can't express how we all feeling rite now but rest assured you will never EVER be forgotten!!!!! Love always and forever Matt Phoenix Faith!!!!!!"
Ms O'Brien's family in England, where she was from, could not be reached for comment last night.
- James Ihaka, NZ Herald