Laws of Cricket 1947 Code - Law 14
The side which bats first and leads by 150 runs in a match of three days or more, by 100 runs in a two-day match, or by 75 runs in a one-day match, shall have the option of requiring the other side to follow their innings.
Note on Law 14
1. In Australia the lead necessary to enforce a follow-on in a match lasting three days or more is 200 runs.
Amendment
In September 1951 the Australian Cricket Board of Control agreed that the follow-on margin should be 150 rather than 200. Subsequent to that date, the Note was no longer published with the Laws.
Reproduction of the Laws of Cricket is by kind permission of Marylebone Cricket Club