A contract worker on the SCG redevelopment has been sacked for allegedly smuggling a flare into Allianz Stadium hours before the weekend's A-League derby.

For 90 minutes on Saturday evening, Allianz Stadium vibrated amid the noise and colour of 40,388 passionate fans who turned the venue into a sea of blue, red and black.

A security guard handles a flare during the A-League derby between Sydney FC and the Western Sydney Wanderers on Saturday. Credit:Getty Images

But it was the orange tinge of two flares lit inside the Western Sydney Wanderers supporters' bay that caught the eye of authorities.

Police and security had searched the stadium before the match for any banned objects but still two flares were let off during the game in the south end of the venue, which was occupied by Wanderers' fans, along with several ''bungers'' - small fireworks that give off a loud sound.

Security officers were later shocked to find that a supporter working at the SCG had allegedly misused his position to bypass authorities.

A private building contractor had his employment terminated on Monday after allegedly sneaking inside and hiding a flare inside piping beneath seats reserved for the Wanderers supporter group.