Nearly 600,000 litres of screened but untreated sewage flowed into Halifax Harbour this week after someone broke into a restricted area of the MacKay Bridge.

Alison MacDonald, speaking for Halifax Harbour Bridges, said an intruder entered a secured area of the MacKay Bridge at around 3:30 a.m. on Saturday.

She said the person accessed an electrical panel and turned off breakers, impacting some cameras on the bridge towers, some navigational lighting and a repeater for Halifax Water.

A view underneath the MacKay bridge. (CBC)

That caused 596,000 litres of sewage from five wastewater pumping stations to spill into the harbour over the course of a few hours.

James Campbell, speaking for Halifax Water, said that amounts to about the same as would be deposited in the harbour after a heavy rainfall event.

"It's critical infrastructure so when people tamper with it, it has real implications, so the police are investigating," Campbell said.

MacDonald said the area accessed is restricted at multiple points with gates, fences and is monitored both by cameras and with regular patrols.

"We are taking this seriously and are investigating along with HRP," she said in an email.

"We've taken steps to ensure all panels, even those within secured areas, are locked."

Steve Snider, the general manager of Halifax Harbour Bridges, said someone would have to go "to a special effort to make this type of access on to the bridge."

"We have no idea what the motivation of this individual was, but to do this is foolhardy," Snider said.