Service tax on renting of immovable property

I have a query in regards to renting of Immovable Property. My company has provided a land on lease to a company. The payment of lease were received annually in september every year. And as per agreement, my company issues a single bill for the lease every year. now during service tax audit by department. they are demanding interest for not paying service tax on monthly basis. they consider it in continious supply of service. Department considers it as a default. is it necessary to issue monthly invoice? is it valid?