A young widow has urged anyone with allergies to always carry an EpiPen, following the shock death of her husband after a night out at a Ballarat restaurant.

Key points: The couple were eating dinner when the husband suffered an allergic reaction

The couple were eating dinner when the husband suffered an allergic reaction He had felt a slight reaction seven years earlier when eating the same fish

He had felt a slight reaction seven years earlier when eating the same fish An allergies expert said new allergies could develop at any stage in life

Cassandra Hall was dining with her husband Alexander on February 7 when he went into anaphylactic shock after eating barramundi.

Mr Hall, a 37-year-old who was set to become a father in May, was rushed to hospital but died five days later.

A tearful Mrs Hall told ABC Radio Melbourne that her husband's allergic reaction was "completely unexpected", and left him incapacitated within minutes.

Mrs Hall said her husband had enjoyed his meal and remarked that seven years earlier he had felt a slight tingling on his lips after eating barramundi.

"Between the space of one to two minutes, he just started sweating," she said.

"He was shifting around in the seat. He was noticeably uncomfortable."

Sorry, this audio has expired Listen to Cassandra Hall's full interview with Virginia Trioli

Mrs Hall said she asked her husband if she should call an ambulance, but he could not respond.

"I ran over to the bar staff and I asked them if they had an EpiPen and they didn't. I said 'can you please call an ambulance immediately because I think my husband's having a reaction'," she said.

EpiPens are auto-injecting devices which deliver a shot of adrenaline and are most often used to counteract an anaphylactic reaction.

'I would hate for this to happen to somebody else'

Mrs Hall said she had received overwhelming support since the tragedy earlier this month. ( Supplied: Cassandra Hall )

Despite the best efforts of restaurant staff and two off-duty nurses, Mr Hall suffered significant brain damage by the time the ambulance came and he was rushed to hospital.

"I would just urge people that if you have eaten something once before and you're not 100 per cent sure about it, either get an allergy pen or don't try it again," Mrs Hall said.

"I would hate for something like this to happen to somebody else."

Mrs Hall, who is expecting a baby daughter in May, said she had been surrounded by "overwhelming support".

"She's proving to be quite resilient. She's definitely going to be her father's daughter," she said.

Jo Douglass, who is the director of research at the Royal Melbourne Hospital, described Mr Hall's death as a tragedy.

"New allergies can occur at any time. Usually the most common food people are allergic to is nuts but fish is also a relatively common allergen that can cause anaphylaxis," she told ABC Radio Melbourne.

Professor Douglass said developing allergies in adulthood was rare, but urged anyone who experienced tingling, swollen lips or itching after eating foods to seek medical advice.

An online fundraiser for Mrs Hall and the couple's unborn daughter has received more than $21,000 in donations.