A Sydney gaming startup has been hit by a concerted cyber attack after it offered to support a professional gamer banned from his sport for endorsing Hong Kong democracy protests.

Surry Hills company Immutable was hit on Wednesday by an attack preventing players from logging into its online card game Gods Unchained for about four hours. It came after Immutable announced it would cover prize money worth about $US10,000 ($15,000) that had been withdrawn from a Hong Kong gamer by a competitor.

Robbie Ferguson, left, and his brother James are founders of Immutable. Wolter Peeters

"The stance we were taking was that we don’t think your financial assets should be taken away from you for expressing a belief," Immutable co-founder Robbie Ferguson said.

The online attack shows the risks run by the few companies who take a stance in conflict with China's geopolitical sensitivities, but it also demonstrates the rewards: players angry at Immutable's competitor Blizzard flocked to Gods Unchained.

Blizzard Entertainment - the international video gaming giant behind titles such as Overwatch and World of Warcraft - also makes Hearthstone, an online card game that competes with Immutable's Gods Unchained. This week, Blizzard banned Hong Kong's Chung "Blitzchung" Ng Wai from Hearthstone eSports for a year and withdrew tournament prize money he had won after he expressed support for protests in theChinese territory.

During a live stream of a Hearthstone tournament on the weekend, Mr Chung said, "Liberate Hong Kong, revolution of our times," in Mandarin while wearing goggles and a face mask, the unofficial uniform of pro-democracy protesters in Hong Kong.

Now an Australian startup is coming under a cyber attack.

In contrast to many gaming firms that control the supply and demand of in-game items with real world value at their sole discretion, Immutable aims to give gamers real ownership through the use of cryptocurrency technology.

"We are trying to change players' relationship with gaming and the assets they own," Mr Ferguson said. "If we didn’t do this [supporting Mr Chung] it would be disingenuous or just weak."

Blizzard said Mr Chung's behaviour violated its rules barring players from conduct that "offends a portion or group of the public" or "damages Blizzard image" [sic].

The gaming company Blizzard is part-owned by Tencent, the Chinese internet giant that also led the backlash against the Houston Rockets NBA team after a team official tweeted in support of Hong Kong.

Mr Ferguson said Mr Chung had privately responded on Twitter, saying he appreciated Immutable's offer but did not say whether he would take it up.

Immutable, which counts tens of thousands of players on Gods Unchained and raised $22 million in its last funding round, has much less potential exposure to the enormous Chinese market than other gaming publishers because it relies on cryptocurrency technology. Cryptocurrency trading is banned in China.

About seven hours after Immutable announced its offer, it was hit with the cyber attack that blocked players from logging into Gods Unchained. Mr Ferguson said the attack was continuing but Immutable had managed to ward off the damage after about four hours with the help of external security experts.

While Mr Ferguson said he had not analysed the attack in detail, he believed it most likely originated in China.

Other entities including the NBA and the US sports network ESPN have recently tried to avoid offending Chinese institutions and consumers, with mixed results.

Clarification: Gods Unchained is not banned in China.