Commuters making their way through Perth's new Yagan Square — billed by politicians as the new "heart of the city" — were left shocked after stumbling upon an electronic sign displaying a pornography website.

Where useful information to help pedestrians navigate their way around the city can normally be viewed, the homepage of Pornhub — one of the internet's biggest pornography sites — was instead being broadcast to hundreds of pedestrians on Thursday night.

A spokeswoman for the Metropolitan Redevelopment Authority (MRA) said as soon as it was alerted to the issue, the two-metre interactive screen was switched off.

The sign was turned off after it was "compromised", the MRA said. ( ABC News: Jon Sambell )

She said it was not yet clear what exactly had happened but the MRA was making inquiries as to whether the sign had been hacked.

The MRA later released a statement saying the screen had been "compromised".

"Yagan Square has two touchscreen wayfinding signs, one at each entrance of the William Street Mall," the spokeswoman said.

"Unfortunately, it appears that these screens were compromised this evening and, for a brief period of time, some inappropriate content was displayed on one of the screens.

"Both screens were immediately turned off and power has also been switched off to the units to eliminate all possibilities of a recurrence.

"The MRA takes this matter very seriously and will undertake a full investigation to understand how this event has happened and identify those responsible."

An eye-opening sight

Michael Schmook, 26, was among the commuters who noticed the unusual display.

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He was making his way home from work when he came across the screen about 6:00pm.

"It didn't register at first and I had to take a step back, I was pretty shocked," he said.

"There was quite a few people walking by who took a second glance, some having a bit of a giggle, but a couple looked pretty shocked.

"It's not a huge screen but it's pretty prominent as you walk through crossing from the CBD to Northbridge."

While some took to social media to voice their surprise, it was likely a familiar sight for others, with Pornhub ranking as the 23rd most visited website in Australia last year.

Yagan Square was opened to the public last month after 10 years in the making, connecting Perth's CBD to Northbridge at a cost of $73.5 million.

WA Premier Mark McGowan referred to the project as a new "heart of the city", outlining a hub of the CBD that would invite families, commuters and diners into a new cultural and culinary space.

It includes a park for children, a water feature, an amphitheatre for public performances and gatherings, and a multicultural food hall to showcase WA produce.

Another major feature is a digital tower that screens items of public interest, including sporting matches.