There are two aspects to your question. How hard do you need to work to get a particular grade, and do grades count at all?
Most people find that uni starts slow. The pace is sedate and the subjects easy, basically revision of Y12. They then get blindsided by either a genuinely tough subject, or by assignments with major amounts of work being left too long.
Picking the genuinely tough subjects from the guidebook can be hard. For example, most people find anatomy a nightmare simply because of the huge amount of rote learning. Once you're in the course, students further in the course will let you know
More commonly though, people simply get whacked by poor time management. So if you are finding uni easy, get into the luxurious position of being ahead of the assignment curve. You'll notice the time management of part-timers is much better than full timers, the PTs will spend less than 8 hours on campus, yet few FTs would knock their work over in 16 campus hours.
Do grades count? It really depends.
Firstly, your grades count if they represent a wall, just as your HSC mark allowed you to get over the wall into uni and from then on the HSC signifies less and less. The most common wall is that high grades are needed for entry into honours and the fast track to a PhD. So if you like uni like and wouldn't mind working in one, you need big grades. Some walls are also set by professional associations (eg, high marks sometimes allow you to skip various "registration" requirements).
Secondly, when you have no experience your academic transcript is all your potential employer has to compare with other candidates. So big grades get you into the interview. Obviously with smaller grades you have less choice. This might not matter, you might be in a field where there is a shortage of employees. But don't get too smug in first year. For example, the IT industry turned from undersupply to oversupply between the start and end of a degree.
Thirdly, your grades matter because they reinforce the view that you want to give your potential employers: if you say you are "hard working and self-motivated" but have multiple Withdraws, PassWithoutProceed on your transcript they are hardly going to believe you.
Finally, and most importantly for many, is simply the satisfaction of doing your best at something that you'll never have the opportunity to do again.
[BTW, there is no fixed % of the year that gets entry into honours. If people are good enough, and they apply, and there is space, then they get in. Whatever % this works out to in practice will vary massively by subject and may not mean much (eg, in some courses there are more honours places available than are taken up, so the % reflects student's interests rather than rationing)]
