Police are investigating two separate grass fires at Aldinga Beach, south of Adelaide, this weekend.

The first grass fire flared up on Friday night while the second was ignited on Saturday night.

South Australian Police are speaking with witnesses and have appealed for anyone else with information to come forward.

Some geniuses let off fireworks on the esplanade at Aldinga Beach. Now there’s a fire. After the month we’ve had. #Adelaide pic.twitter.com/NxjZzcVnFK — Daniel Gannon (@DanielGannon) December 28, 2019

The Aldinga Beach Country Fire Service condemned the cause of the blaze on social media.

"It is known that there were illegal fireworks used last night in our local area, in multiple locations," the fire service said in a post.

"This is not acceptable on any level."

"We urge the community to be aware and alert, we have hazardous conditions forecast for the days ahead."

READ MORE South Australian firefighters work around the clock ahead of high temperatures

The CFS is concerned about weather forecasts for Monday when temperatures are likely to hit 40C in Adelaide with strong northerly winds forecast.