Fears for one of Australia's best known equestrian riders after a 'probable' heart attack at the Royal Easter Show as her sister shares harrowing update about three-time Olympian

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Australian three-time Olympic equestrian rider Vicki Roycroft is undergoing urgent bypass surgery after suffering a suspected heart attack as friends and family urge everyone to keep her in their prayers.

The 69-year-old was preparing to take part in a show jumping event at the Sydney Royal Easter Show on Saturday when she suffered a medical episode.

Roycroft was rushed to Concord Hospital before she was transferred to Royal Prince Alfred Hospital's intensive care unit, where doctors won't know the extent of the damage for several days. 

Champion showjumper Matt Williams shared a harrowing update from Roycroft's sister Joanne late Saturday night.

Vicki Roycroft (above) was rushed to hospital after a 'probable' heart attack on Saturday

'Vicki has been stabilised at Concord Hospital and transferred to Royal Prince Alfred Hospital,' the post reads.

'She has a large tear in her aorta. She did not breath for 10 minutes so they do not know what damage has been done. 

'She is presently having an eight-hour operation for bypass surgery.

'Please all pray for Vicki, it will be three to four days until they know the damage that has been done. Doctors are amazed with her fitness, so this is a plus.'

Mr Williams added his own message for Roycroft, writing: 'If anyone can get through this you can. [You're] one of, if not the, toughest person I've met in this sport. 

'You're a inspiration to us all and we're all praying for you.'

Fellow champion show jumper Matt Williams (right) shared an update on Vicki Roycroft's condition (centre) on Saturday night

It is understood Roycroft suffered the episode while preparing for the show jumping competition at the Royal Easter Show.

'The on-site St John Ambulance team attended to her immediately, with treatment also provided by NSW Ambulance paramedics,' a spokeswoman told the Sydney Morning Herald.

'The thoughts of the entire Royal Agricultural Society of NSW and horse community at the show are with Vicki and her loved ones at this time.'

Fellow equestrian Claudia Studdert, who used to take lessons from Roycroft was among the many well-wishers.

'She is the reason I even began jumping and has added countless amounts of wisdom and inspiration into my life that I still take into account regularly,' she said.

'Not to mention she is the reason I have my Barbie pony (who is my favourite thing in this world) and of course the reason I have Laz the legend.

'So much love Vicki, please get well soon. We are all praying for you.'

Vicki Roycroft (above) represented Australia as a member of three Olympic and three World Championship teams

Another wrote: Praying you pull through this Vicki Roycroft, Strongest woman I know Such a role model to me and so many others.'

Roycroft represented Australia at three Olympics and three World Championship titles.

She was named Australian Champion on three occasions - in 1983 for three-day eventing and for show jumping in 1996 and 2005. 

Roycroft was among the first to receive the Australian Sports Medal, in 2000, awarded by the Governor-General in recognition of Australian sporting achievements.

She was married to fellow three-time Olympian Wayne Roycroft for 24 years before they split in 2000.