A high-achieving Sydney mum-of-three who built a successful career in tech has died suddenly just weeks after giving birth, with her family now urging women to seek medical advice for seemingly minor symptoms.

Cathy MacGuinness, 40, suffered a cardiac arrest at her Marrickville home on Easter Sunday, just two weeks after giving birth to her third daughter, Goldie.

Her husband, Sebastian Fletcher, performed CPR before paramedics arrived and took over, restoring a pulse before she was rushed to Royal Prince Alfred Hospital, where she was placed in an induced coma.

Despite efforts to save her, MacGuinness died on April 25 due to severe brain damage after being without oxygen for too long.

She leaves behind Fletcher and their three daughters — Zoe, 7, Rosie, 3, and newborn Goldie.

Jo Allen, MacGuinness’s best friend who created a GoFundMe page to support the family, told 7NEWS.com.au MacGuinness had built a successful career, most recently serving as Head of Operations at Alauda, an aerospace company, after working at Microsoft and Commonwealth Bank.

“She’s absolutely irreplaceable and we’re never going to find anyone like her again,” Allen said.

“She made everyone feel like they were important — that’s a really special trait for someone to have.”

In the fundraiser, Allen described her as someone who embraced life’s challenges head-on, “climbing whatever Mt Everest presented her with that day”.

“She shows up with her full self, always. She lights up any room she walks into — magic in human form,” Allen wrote.

“She reads the card you gave her on the spot, just to double the love in real time. She travels across the city to walk a friend to work. She drives you home at midnight. She buys the drinks despite not drinking any. She dances like no one’s watching.

“She accomplishes everything she sets her mind to, looks incredible doing it and still makes everyone else feel a million bucks at the end.”

“She’s also a cool mum with a nose ring… how is she possible?!”

Allen said MacGuinness’s greatest love was her family — especially her three daughters, who meant the world to her.

MacGuinness’s sisters-in-law, Gillian Binchy-MacGuinness and Alison Scotland, said losing her was “the hardest thing” they had ever gone through.

“We said goodbye to the brightest spark ever,” they told 7NEWS.com.au, describing her as “magic in human form”.

They said even the midwife who delivered Goldie later messaged her partner, saying: “This woman has just made my day.”

“She could make that kind of impact so quickly with people,” they said.

The pair, who were with MacGuinness in hospital, described the three weeks as a “roller coaster”, clinging to hope for a miracle that never came.

“Those weeks were horrible, but we really don’t want that to be the focus,” they said.

Instead, the family wants MacGuinness’s death to serve as a warning to other women not to ignore seemingly minor symptoms.

They said she had an undiagnosed heart condition known as dilated cardiomyopathy, which causes the heart to enlarge and struggle to pump blood effectively.

Symptoms can include fatigue, dizziness and swelling — signs they said are often easy to dismiss, particularly for busy mothers.

“If she had been diagnosed and on the right medication, none of this would have happened,” they said.

They are now urging women — especially new and young mothers — not to put their own health last.

“It’s so easy to dismiss yourself or just be like, ‘oh, I’m just tired’. Or, ‘yeah, I’ve been on my feet all day, that’s why my legs are swollen’, and not to give it a second thought because as a mother, you put everybody else in front of yourself,” they said.

“Don’t dismiss even the smallest symptoms… go and see your GP.”

The fundraiser has raised more than $110,000 in 24 hours, with over 400 people contributing.

The family said the outpouring of support reflects just how many lives MacGuinness touched.