Train services have resumed but traffic snarl-ups are expected to continue after explosions and fireballs closed a rail overbridge in south Auckland.

The explosions were reported just before 8am on Wednesday at the Weymouth Road overbridge in Manurewa.

The explosions closed the road and temporarily stopped trains in the area, but Auckland Transport later in the morning said trains were up and running again.

However, adding in a crash, a detour around the overbridge and power cut to nearby traffic lights, significant traffic delays and disruptions were expected.

Lines company Vector said the explosions were the result of an 11kV cable fault, which cut power to nearly 2000 customers.