It turns out climbing the arch of a major Brisbane bridge in high heels to take a selfie is a bad idea.

A young man and woman have been fined after making the "crazy", "stupid" trip up the arches of the William Jolly Bridge in the CBD on Wednesday morning.

But the traffic reporter who captured their escapade said they were lucky the result hadn't been worse.

Brad Hunter said he almost fell off his scooter when he saw the duo atop the bridge, casually taking photos of themselves and each other about 10.30am.