Mont. Code Ann. § 7-13-3024

A special assessment made and levied to defray the cost and expenses of the work for the system, together with any percentages imposed for delinquency and for cost of collection, is a lien upon the property upon which the assessment is made and levied, from and after the date of the passage of the resolution levying the assessment. The lien may only be extinguished by payment of the assessment, with all penalties, costs, and interest.