Speeding fines in WA as at 1/1/2010:

Less than 9 km/h - $75 0 Demerit Points
10 km/h - 19 km/h - $150 2 Demerit Points
20 km/h - 29 km/h - $300 3 Demerit Points
30 km/h - 39 km/h - $700 5 Demerit Points
More than 40 km/h - $1000 7 Demerit Points

So exactly 20 would be $300 and 3 demerits.
They will generally reduce your actual speed by 1 or 2 so you might be lucky.