A 47 year old woman was taken to hospital in a serious condition during her performance on 10 March.
Adele stopped mid-performance telling fans she didn't want to shock the sick woman with fireworks that were planned.
She also joked that she had ruined the surprise for the rest of the audience by telling them about the pyrotechnics.
"Sorry. Someone got hurt. I ruined the surprise for you didn't I?" Adele told the crowd.
"Sorry, there was so much going on it frightened me. Pretend I didn't say anything. Let's start again."
She continued the show after a brief pause.
An eyewitness told local news that it took staff some time to realise that the woman was seriously ill.
"Security staff were trying to rouse her because they assumed she fainted," Adele fan Daniel Lowe told 9news.com.au.
"It took a full song for the actual on-site medics to respond.
"They then started performing chest compressions for a good 15 to 20 minutes until ambulance officers arrived with proper equipment."
Fans said that the ill woman had been sat in the front row at the Sydney show.
Daniel also added that Adele seemed "visibly upset for the rest of the show".
"She wasn't as perky and smiley as she was as before," he said.
Footage online shows Adele firing t-shirts from a cannon on the stage (plus $20 for the person who caught it to get themselves a drink on her), but it is not clear whether this happened before or after the fan had a heart attack.
Adele performed two shows at Sydney's ANZ Stadium as part of her world tour, which started over a year ago on 29 February 2016.
She returns to the UK for four shows in late June and early July before the tour comes to an end.