If the court is satisfied from the evidence that the assessment is erroneous and that the erroneous assessment was not caused by the wilful failure or refusal of the applicant to furnish the tax-assessing authority with the necessary information, as required by law, the court may order that the assessment be corrected and that the applicant be exonerated from the payment of so much as is erroneously charged, if not already paid. If the tax has been paid, the court shall order that it be refunded to the taxpayer, with interest at the rate provided by § 58.1-3918 or in the ordinance authorized by § 58.1-3916, or as otherwise authorized in that section. If, in the opinion of the court, any property is valued for taxation at more than fair market value, the court may reduce the assessment to what in its opinion based on the evidence is the fair market value of the property involved. If, in the opinion of the court, the assessment be less than fair market value, the court shall order it increased to what in its opinion is the fair market value of the property involved and shall order that the applicant pay the proper taxes. For the purpose of reducing or increasing the assessment and adjusting the taxes the court shall have all the powers and duties of the authority which made the assessment complained of, as of the time when such assessment was made, and all powers and duties conferred by law upon such authority between the time such assessment was made and the time such application is heard.