In case it is purely a liquidity problem, which is a relatively idyllic case, the central bank plays a role as “the lender of last resort” based on classical Bagehot’s principle.
The second would be sharp currency movements that seriously threaten the conditions that support sustainable, long-term economic growth. In a market breakdown, intervention would aim to restore market functioning and would likely be quite short-lived. In extreme currency movements, it’s understood that intervention would be part of a broader set of policy changes to re-establish confidence in Canadian economy.