Asics New Zealand said that an unknown person had gained access to its televisions. (Representational)

The New Zealand arm of Japanese sports brand Asics has apologised after pornography was broadcast on large television screens above its central Auckland store for several hours, startling Sunday morning shoppers.

Pornography played on the retailer's outdoor screens for up to nine hours -- until staff arrived at the shop around 10am on Sunday and switched them off, the New Zealand Herald reported.

"Some people were shocked, but others just stopped and watched," security guard Dwayne Hinango told the newspaper.

The explicit content was "totally inappropriate and offensive" and "not something you want kids exposed to", said Tanya Lee, who had walked past with her seven-year-old son.

Asics New Zealand said in a Facebook post that an unknown person had gained access to its televisions.

"Some objectionable content was displayed on the screens," the post said. "We would like to apologise to anyone who may have seen this."

The company said it was working with its software and online security suppliers to ensure the situation was not repeated.