A tax period is the length of time for accounting for GST on an entity's activity statement and is either a month or a quarter. An activity statement must be lodged for each tax period. Therefore a BAS must be lodged either monthly or quarterly.
Quarterly tax periods are periods of three months ending on 30 September, 31 December, 31 March and 30 June.
Monthly tax periods end on the last day of each calendar month.