Sunrise host Nat Barr breaks down live on air after learning that she knew one of the victims of the Sydney siege, 38-year-old mother Katrina Dawson. Courtesy: Sunrise/Channel Seven

SUNRISE host Natalie Barr has broken down in tears on air this morning after learning she knows the brother of the woman killed in last night’s siege.

With her voice shaky and tears in her eyes, Barr revealed that 38-year-old Katrina Dawson’s brother is a lawyer who has worked for Channel Seven.

“I’m just finding out Katrina Dawson was the sister of a prominent barrister Sandy Dawson who has done some work at Channel Seven,” Barr said, almost struggling to get the words out.

“Sandy Dawson, who I know and I have friends who know. A mother of three children. I’m just finding this out this morning.”

“When you put the faces to the people who have died overnight, it’s just heartbreaking,” she added.

Ms Dawson, a barrister and mother of three children, aged eight, five and three, entered the Lindt Cafe yesterday morning to grab a coffee, reportedly part of her daily routine.

She and a colleague from their law firm Eight Selborne, who is believed to be midway through her pregnancy, were held hostage inside the cafe for 17 hours.

A visibly shaken Barr was given a few minutes to compose herself before continuing the broadcast from a street near Martin Place.

Viewers who were moved by Barr’s distress immediately took to Twitter to sympathise with her and congratulate her on her composure.

Heartbreaking @natalie_barr as she finds Katrina Dawson is sister of a friend Reporters doing amazing job @theheraldsun @sunriseon7 #siege — Susie O'Brien (@susieob) December 15, 2014

One of Australia's best morning newsreaders is @natalie_barr You laugh /and/ you cry! And I cry with you. Thankyou for being so real. xox — Bianka Dillion (@home_town_girl_) December 15, 2014

.@sunriseon7 sad to see @natalie_barr cry on tv as she discovers she knows the sister of one deceased hostage- Sorry Natalie — Kayleen Bell (@kayleen_bell) December 15, 2014

@natalie_barr your tears made me cry ...I hope you are ok :) — Nicky Byrne (@Nixwitch) December 15, 2014

Natalie Barr crying just breaks my heart. Shows what a truly tough 17 hours it has been in Sydney, for all involved #sun7 #sydneysiege — Alexandra Agrio (@thefreeddove) December 15, 2014

@natalie_barr Stay Strong Nat — Rebecca Davis (@BBBBBEEEECCCC) December 15, 2014

All Seven staff were evacuated form their Martin Place headquarters which were situated directly opposite the Lindt cafe at 10.20am yesterday morning, according to Seven reporter Chris Reason.

They remain locked out from the building.