A young mother has died unexpectedly in her sleep at just 25 years old leaving her devastated family looking for answers. 

Ruby-Lee Arnott, from Victoria's Mornington Peninsula, died overnight on July 7, leaving behind three children and her partner who was planning to propose on their anniversary just a few days later. 

'We're still trying to come to terms that it's real,' Ruby-Lee's mother Stacey Arnott told Daily Mail Australia. 

She said Ruby-Lee and her partner Korey had been planning on visiting the Dandenongs where he was going to pop the question with a custom ring he had made.

'It's been very difficult the last few days,' Ms Arnott said.

'Ruby was really such an amazing mother. I was so proud of her, she had so much patience with the kids and did so much with them.

'We don't have any answers yet, which of course makes it harder not knowing,' she said, adding that the family is awaiting the results of an autopsy.

'She had a cold for a few days, so did her partner and kids, and she complained of a sore chest two days earlier to her cousin.'

A GoFundMe has been launched to help Ruby's family with funeral costs. 

Friend Angie Galbraith said on the fundraiser that Ruby was a 'vibrant and loving individual who touched the lives of everyone she met'.

'We are heartbroken to announce the sudden and tragic passing of our dear friend,' she said.

'She leaves behind three beautiful young children Amelia, seven, Chase, five, and Violet, 10 months, who will forever cherish the memories of their wonderful mother.'

She said any additional funds after the funeral expenses would be used to provide for her children. 

Another friend Trish Scarlett said the pair formed a friendship at a young age when they were in a sandpit and Ruby made her laugh by pointing out some cat poo.

'Her becoming a mum was all we ever spoke about as kids and, for her to become a mum, it was so beautiful to see,' she told The Herald Sun.

'Her kids are an absolute tribute to how she was as a person.'

She described her as 'unique' and a 'rock' and said she would dearly miss her friend.