Quakers Hill Superintendent Mark Cooper said three disguised men carrying knives entered the Punjabi Supermarket in Railway Road, Quakers Hill, at 9pm on Monday and threatened the lone male employee, who was closing the shop.

The robbers stole money from the cash register before splashing accelerant from a bottle onto the floor, shelves and stock and igniting the petrol.

Police allege the accelerant and smoke caused an explosion and the employee and door was blown through the front entry of the shop, onto the footpath outside.

The employee, 33, was taken to Blacktown Hospital where he was treated for minor burns and smoke inhalation and released at 3am.

The fire damaged stock and the structure of the premises, which is about 100 metres away from Quakers Hill police station.

``I wouldn't describe it as brutal,'' Superintendent Cooper said.

``But we have to take it seriously, it could have been fatal.''

Superintendent Cooper said the fire may have been planned and lit to cover up forensic evidence at the scene.

He said there was ``no element of racial tension'' to the crime, but acknowledged the Indian dvd shop next door to the supermarket had been robbed and set alight by men allegedly wielding knives in an unrelated incident about three months ago.

A nearby shopkeeper who did not wish to be identified said he was locking up last night when he heard a loud bang.

``I ran outside and saw the man on the ground. I saw there was smoke coming from the shop and came inside to call the police,'' he said.

``That place has been robbed a few time in the past few years. The employee is a really nice man, he is a Sikh Punjabi and has been there for about two years.''

The shopkeeper said he had installed closed circuit television cameras, and was feeling ``cautious and worried'' about a similar attack on his business.