I married the man of my dreams who accepted my two sons as his own. Then my world came crashing down when he suddenly died 18 months later

A much-loved Aussie dad who died unexpectedly just 18 months after getting married is being mourned by his new wife and their three young children.

Jake Mumberson, from Gledswood Hills in south-western Sydney , died after suffering a heart attack on November 1 - just 11 days before his 32nd birthday.

He and his wife Cassie, 29, had just gotten married 18 months earlier when he became the father of her two sons, Zak and Nix, from a previous relationship.

'They called him Dad,' Ms Mumberson told Daily Mail Australia on Tuesday. The couple also share a two-year-old daughter, Willow.

Mr Mumberson, who ran his own fencing and landscaping business, was in hospital for an unrelated condition when he died last week.

'Two hours after being there, he collapsed and went into cardiac arrest,' Ms Mumberson said. 'It was just completely out of the blue.'

Tragically, Jake had previously had his heart checked after his mother died of a heart attack three years ago but 'everything came back normal'.

Ms Mumberson said it was 'terrible' having to break the news to their children.

'Jake's grandmother also passed away last year and the kids loved her so much,' Ms Mumberson said.

'So we've just pretty much said to them that he's now in heaven with her and his mummy. That's how we told them.'

She said the younger children don't really understand that their dad is gone.

'My daughter does not, she's two. My son who's six, he does understand but doesn't at the same time. But my nine-year-old, he does. He definitely does.'

Ms Mumberson said she is still coming to terms with her husband's death.

'I'm still in a lot of shock. I'm on a roller coaster of emotions. I am sad and then I cry and then I'm happy and then I don't know, it's just all over the joint,' she said.

The sudden loss has meant Ms Mumberson has not yet made plans for a funeral.

To help the family out during a time of incredible stress, family friend Millie Jadrijevic has set up a GoFundMe .

'Jake was unexpectedly and unjustly separated from his beloved wife Cass, Zak and Nix who he treated wholeheartedly like his own, and beautiful daughter Willow,' Ms Jadrijevic wrote.

'Their journey and the next chapter of life were abruptly cut short.'

Ms Jadrijevic said Mr Mumberson was a 'devoted young husband, father, son, brother, and cherished friend'.

'As Cass grapples with the task of forging a future without Jake by her side, life feels daunting and uncertain,' she wrote.

Ms Jadrijevic said her friend now has to 'navigate the loss of her husband while striving to provide the very best for her three beautiful children'.

'Every single donation makes a difference to support Cass on this path to healing, allowing her the necessary time to regain her footing,' she added.

'Fly high, Jake. Watch over Cass and your precious children.'