The partner of a mother killed while saving her baby from a stabbing rampage in a busy Sydney shopping centre has spoken out for the first time, saying the day she died will “never make sense”.

Ashlee Good, 38, was among six people killed by Joel Cauchi at Westfield Bondi Junction on 13 April. Her daughter Harriet, who was nine months old at the time of the attack, was also injured.

Australian authorities say the stabbing was “mental health” related, but they believe Cauchi targeted women.

Thanking those who had sent donations, Good’s partner Dan Flanagan said “the overwhelming support, kindness and generosity” his family had received has given them “strength”.

Mass killings are rare in Australia, and the attack – at one of the country’s biggest and most popular shopping centres – stunned the world.

Horrified witnesses have described how Good fled into a nearby store, desperately seeking help for her injured baby girl.

In a post to a GoFundMe page, which raised almost A$850,000 (£439,000;$560,000), Mr Flanagan said the loss of his partner – an osteopath who friends described as “full of life and love” – was “unimaginable”.

“The day our family of three became a family of two will never make sense to me, but countless people… have shown me that while tragic things do unfortunately happen, there is more good in this world, than bad,” he wrote.

“Nothing will ever dull the pain of losing Ash, but reading your messages and seeing what an impact Ash has had on so many people has provided a lot of comfort.”

He also sent his condolences to the other families, friends and communities dealing with the trauma of the attack.

“Our lives will never be the same, and I wish every one of you your own strength and hope you’re getting the support required to navigate such incomprehensible and difficult times,” he said.

Cauchi stabbed 17 people before he was chased down and shot by a lone police officer.

Along with Good, five others died – security guard Faraz Tahir, 30; Jade Young, 47; Pikria Darchia, 55; Dawn Singleton, 25; and Yixuan Cheng, who was believed to be in her 20s.

Baby Harriet – the youngest victim targeted – underwent surgery to treat her chest and arm injuries to survive.

A special strike force was created to investigate the incident, with New South Wales Police saying it could take months to present its findings.

Cauchi’s parents said the 40-year-old had battled with mental illness and had come off his medication shortly before carrying out the attack.

Some, including the family of Jade Young, have called for an immediate boost to mental health services to prevent similar violence from occurring in the future.