Police are investigating a brawl involving more than 20 tradies outside a building site in Sydney’s north-west.

A massive brawl involving more than 20 tradies erupted outside a building site in Sydney's north-west this morning.

A massive brawl involving more than 20 tradies erupted outside a building site in Sydney's north-west this morning.

A massive brawl involving more than 20 tradies erupted outside a building site in Sydney's north-west this morning.

The subcontractors were sacked from a Meriton building site in the city’s north-west. Days later, police allege they returned armed with hammers.

The subcontractors were sacked from a Meriton building site in the city’s north-west. Days later, police allege they returned armed with hammers.

Footage has emerged of a violent clash between more than 20 tradies at a construction site, prompting a police investigation.

Footage has emerged of a violent clash between more than 20 tradies at a construction site, prompting a police investigation.

Punches and kicks are thrown as the violence spreads down the street, with some tradies using their hard hats as weapons.

Punches and kicks are thrown as the violence spreads down the street, with some tradies using their hard hats as weapons.