A teenager has been charged after police on patrol in Sydney's west allegedly found materials that could be used to make an explosive device inside his backpack.

Police from Public Transport Command stopped the 18-year-old from Telopea about 10am on Wednesday, while patrolling on Church Street, Parramatta.

Police allege the man's backpack also contained a number of items stolen from the nearby Westfield. Credit:Google

A subsequent search of the man's backpack yielded "items which were consistent with the manufacture of an IED", police said in a statement.

Officers also allegedly found jewellery, a mobile phone case and earphones inside the bag. Further inquiries revealed the items had been stolen from a store in the nearby Westfield shopping centre earlier in the day, police said.