The entire shoreline of a Southern California city is closed to swimmers after sewage-contaminated runoff flowed into California from Mexico's Tijuana River.

The San Diego Union-Tribune reports that the San Diego County Department of Environment Health issued the order Sunday.

They are expanding on the closure that has been in place for months for part of the Imperial Beach coast - which now encompasses the whole shoreline in the city.

Officials say more than 110 million gallons (416 million liters) of toxic storm water has flowed north from Mexico since April, making the picturesque spot dangerous to swimmers.

Authorities often are forced to close a stretch of beaches near the border because of sewage contamination from the southern Tijuana River. The signs used are pictured at Imperial Beach

The southern part of the coast along the Tijuana Slough National Wildlife Refuge has been closed since November last year.

The national park straddles the US-Mexico border with a large metal border fence reaching out into the sea along the coastline.

The waterways are connected by the 193 km-long Tijuana River which has carried the affected water north with the river mouth at the north Pacific coast the affected area.

A heavily polluted section of shoreline, contaminated by water from the Tijuana River

The closure will be in place until testing shows the water is safe.

Signs warning people of the contaminated water have been placed on beaches as the 'continuing' problem infects the picturesque coastline.

The signs read 'exposure may cause illness', but some people continue to take a dip regardless.

California and the cities of Imperial Beach and Chula Vista sued the Trump administration last year over the recurring toxic flows from Mexico.

The shoreline in Imperial Beach, California. It is connected to Mexico by the Tijuana River

Chula Vista is situated on the San Diego Bay, just north east of Imperial Beach.

The lawsuit seeks to force the U.S. government to upgrade the collection system that diverts flows to a wastewater treatment plant.

In January, high tides saw the city's streets flooded with raw sewage from Mexico.

'It's hard to sort of talk about how polluted this water is,' Mayor Serge Dedina said at the time. 'It's raw sewage. It's the worst stuff you could possibly imagine.'

The mayor said the Trump government seems to be fine with his city and surrounding areas 'being the open sewer for Tijuana.'