Witnesses say the victim was hit flush on the face by an Ellyse Perry six, after the ball diverted its course when it skimmed the top of a chair and hit the child near the Ridge Street entrance to the ground.

Perry (91 not out) immediately left her crease and ran over to the crowd to check on the boy who was conscious, but dazed. Medical staff tended to the boy with ice packs before paramedics arrived.

He was able to walk into an ambulance, which took him to hospital for further observation.

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"It's never nice to see someone get hit in the head particularly with a cricket ball, I was really concerned because it hit him pretty hard," Perry said.

"I hope he's OK, he seems like a tough cookie and his parents and medical staff were around him looking after him but it's not nice to see.

"I just caught sight of it when it landed, hit the concrete and kick up and hit him in the face. It's coming pretty hard and it's a hard ball so I think your immediate reaction is to see if he's OK."

The incident came during Saturday's record-breaking Big Bash double header in front of 3914 fans who witnessed Gardner's magnificent 47-ball ton, which eventually came to an end at 114 after she'd bludgeoned 10 sixes.

It was the biggest ever individual score in the WBBL, only the third century in the league's history, and the most sixes ever hit in an individual innings in this competition.

Gardner's knock was the equal second highest score in Big Bash history, level with Ben McDermott's ton last season and behind only Luke Wright's 117 six years ago.

"It's so surreal, I can't believe it's happened, to be honest," Gardner said.

"When I got out there Pez [Perry] just said to me bat the way that you do and it did help that they bowled into my arc probably more times than not.

"It was so awesome to be out there and obviously to score a hundred, only the third one in the WBBL is just so special.

"I'd love to do it again, first game so hopefully I can put on another good performance throughout the year."

The Sixers finished with 4-242, a domestic T20 record in Australia. It bested the Hobart Hurricanes BBL record of 8-223 last season, and was higher than the 7-233 Victoria made against New South Wales in 2006.

Their performance marked a perfect start to their WBBL title defence, and gave them their first ever win over the Stars.

Earlier in the day the Sydney Thunder had set a new WBBL mark for the highest team score when they posted 6-200. They owned the record for barely four hours.

Gardner and Perry put on 150 runs for the second wicket after the Stars had won the toss and elected to bowl.

And Perry was happy to play second fiddle to the young Australian all rounder who made a mess of the visiting bowlers, constantly dispatching the white ball over the fence.

One such six carried 89m and landed on the roof of the stand near the Ridge Street entrance, not far from where the young fan was later hit in the head.

Once Gardner holed out to Alana King in the deep off Emma Kearney's bowling, Perry stepped up and commanded North Sydney Oval once again. Last month she scored an unbeaten 213 here in the Women's Ashes Test match.

She finished not out again with four sixes and nine fours, setting the Stars a near impossible task.

Melbourne began their run chase aggressively with Lizelle Lee and Alana King putting on 61 for the first wicket. Once the Sixers broke through, steady wickets began to fall and Lee (64) was the only one to pose a serious danger.

Marizanne Kapp and Sarah Aley each took two wickets for the Sixers.