Where the Commissioner is satisfied that any amount of tax has not been paid or has not been paid in full, the Commissioner may make a reasonable estimate of the unpaid amount and issue to the retirement fund or insurer concerned, as the case may be, a notice of assessment of the unpaid amount: Provided that the Commissioner may not issue an assessment after 31 August 2008 unless the Commissioner is satisfied that the fact that such tax was not paid or was not paid in full was due to fraud, misrepresentation or non-disclosure of material facts: Provided further that the Commissioner may not issue an assessment under any circumstances after 28 February 2010.