Hundreds of people have been evacuated after a fire broke out at a popular RSL club in Southern Lake Macquarie.

Fire crews from three stations were called to the Doyalson Wyee RSL on the Pacific Highway at about 8.30pm on Wednesday.

They discovered a fire in a microwave oven in a baby change room.

400 people were evacuated as crews tackled the fire.

Four people were treated by paramedics for smoke inhalation, with one taken to hospital for further treatment.

The fire caused only minor damage to the room.