In 1840, when the Treaty of Waitangi was signed, cars were in the future. This is how I remember that ka, as a tense marker, indicates the future.
Ka haere au.
I will go.
Ka hīkoi ia,
He or she will walk.
However, ka doesn't always refer to something in the future. Ka is often used in story-telling to say what happened next. Ka relates to subsequent action. If we don't specify the time, then anything subsequent to now is in the future, which is why ka haere au means "I will go".
But when we are telling a story, we often say when it happened. "Last week, I went to town", or "yesterday, the kids went to school." And, once we have established the time, in the past, and want to say what happened next, then we can use ka.
For example:
I tērā wiki i haere au ki te toa, ka hoki kai au.
Last week I went to the shop, and then, I bought food.
So, first we establish when the story starts: i tērā wiki - "last week". So we begin with the past tense marker i. But, since we know now that we are talking about a past event, when we want to say what happened next, we use ka.