It's going to depend on the situation. You can think of situations where it will take $1,000 more earnings, that they will owe.... Let's take your assumption that they have exhausted their taxable..., so their income is above the sum of their non-refundable credits. Then they are going to pay more tax, starting at a pretty low rate. For example, in Ontario it would be 15% federal and just above 5% provincial. If they are in the phase-out rate for the proposed Canada workers benefit, there will be a 12% reduction in their entitlement.
On April 24th, 2018. See this statement in context.