Limitation on tax assessment: time limits restrict reassessment, extended for concealment and for tribunal-triggered reassessment. Section 34 limits the Commissioner's power to make or re make assessments to a statutory period measured from either the end of the year containing the tax period for which a return was filed or the date of a prior assessment, with an extension where tax appears unpaid due to concealment, omission or failure to disclose material particulars; additionally, assessments required by a tribunal or court decision may be made within a further period after that decision.