It was meant as a fun Christmas promotion — but things went terribly wrong.

Key points: A balloon drop, with gifts and prizes, caused a crowd crush at Westfield Parramatta at about midnight

A balloon drop, with gifts and prizes, caused a crowd crush at Westfield Parramatta at about midnight Five people were taken to hospital with injuries and chest and back pain

Five people were taken to hospital with injuries and chest and back pain Scentre Group, which owns Westfield, said it was investigating

Five people were taken to hospital after a Westfield Parramatta event billed as the "ultimate mega balloon drop" resulted in a stampede as the crowd raced for free gifts.

Balloons were filled with gifts cards and other prizes and dropped at the shopping centre in Sydney's west shortly after midnight.

Several people were knocked off their feet, with five taken to hospital.

Injuries sustained included traumatic chest injuries and neck, leg and back pain.

Five people were hospitalised after a Christmas promotion went wrong. ( ABC News )

Hundreds of shoppers had attended the event, which Westfield advertised as a promotion to "keep the party going throughout the night".

It happened as part of a 36-hour trading marathon ahead of Christmas.

Scentre Group, which owns the Westfield chain in Australia, said it was investigating the incident.

"The safety of our customers, retail partners and people is our priority," a spokeswoman said.

"Our team acted swiftly to support our customers, contact emergency services and make the area safe.

"Our team are continuing to speak with affected customers directly."

Retailers expressed surprise over the organisation of the event.

"It was pretty insane. It's been the talk of the morning," one said.

"I think it was a bit over the top. It could have been controlled better.

"The concept is good, it just could have been managed and policed a bit better."