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Two dead, three hospitalised in Taupo district house fire
The woman - understood to be Bruce's heavily pregnant older sister - is recovering in hospital.
Police are investigating the cause of the fire.
Baby Ash was born on December 4 and in a Facebook post Bruce described him as "a miracle baby".
"He's finally here after so many troubles," she wrote.
"My son Ash David Millar you are my world plus more."
Days before her tragic death Bruce posted a heartfelt message on another social media profile.
"I absolutely love this little boy more then I could ever express.
"As a mother we are to teach our children the way of life - but we don't give enough credit to our children for they are the ones who are constantly teaching us... So thank you Ash David Millar for teaching...
"I am constantly running on empty to provide for him, always being hungry because you are fed and cared for first, and having bugger all sleep to ensure you have enough.
"But as a mother its so worth it, to see you happy and healthy. Love you my angel."
In January Bruce entered Ash in a "cutest baby" competition online and said he was an "awesome little man and makes me proud every second of every day".
Millar posted a photograph of himself holding his son on social media in February, prompting Bruce to comment "my perfect little family".
Bruce's friends paid tribute to the young family on Facebook.
"RIP Katie Rose Bruce and bubba Ash and his dad Scott... My love to Susan an her lil family... be strong sweetheart there's so much love and support out there waiting for when you come out of hospital," one woman posted.
Another wrote: "RIP Katie Rose Bruce, RIP baby Ash, RIP Scott Millar. You three were taken too soon.
"My heart goes out to the friends and fams... rest easy angels."
In other tributes, one friend said Bruce was "doing awesome for herself".
"She was doing a good job being a mum and becoming an awesome person," one friend wrote.
"Now, f**k man.. it actually makes you realise to take life serious."
Another friend said Bruce would never be forgotten.
"You will always be in our hearts and souls," she wrote.
"You touched so many lives especially my kids lives when you helped look after them... May you and your beautiful family rest in peace now sweetie.
"You had a whole life ahead of you, you became a mum six months."
The Rangitaiki community were rallying around Bruce and Millar's families.
Police said other members were at the house during the fire and were being supported by officers and Victim Support.
A good friend of Susan Bruce was appealing to the community to donate clothes and shoes for the family, who have lost everything in the blaze.
Bruce is originally from Wanganui and had been working as a farmer until she became a mum.
Millar works in forestry and was a very proud dad.
"Finally have my son," he wrote on an online profile.
Bruce described him as an "awesome daddy".