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 It came after Immutable announced it would cover prize money worth about $US10,000 ($15,000) that had been withdrawn from a Hong Kong gamer by a competitor.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. Credit: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.