The default is, if they're grants and contributions dollars, or if it's above 5% of operating or 20% of capital, it goes back into the general coffers. The responsible department can then make a case for saying they'd like to re-profile that money—that means spending it in a future year—and then the government has a decision to make, but there's nothing automatic.
The default is, if nobody does anything, it goes back into general coffers. If there's a decision made to delay the spending to future years, that's more a fiscal discussion that has to be had.