Just to clarify some of the percentages that other members have provided above, with accurate ones, here is the informatin from AMC:
AMC Awards/Certificates
It should also be noted that awards up to the level of cash prizes are decided by comparison only within the region and year level of the student. For example a student in a certain year in an Australian state or another country will only be compared for the purpose of a certificate or prize with other students in the same group. As a result there will be different cutoffs for awards between Australian states (which each have their own curriculum) and various countries. These cutoffs are not published because they may be prone to misleading interpretations.
PRIZE
A prize is awarded generally to no more that 1 student for every 300 students within their region and year group, the value of a prize depending on issues such as sponsorship money available.
HIGH DISTINCTION
A High Distinction Certificate is awarded to a student who has not received a higher award but is in the top 2% of their year and region (top 5% for Senior).
DISTINCTION
A Distinction Certificate is awarded to a student who has not received a higher award but is in the top 15% (25% for Senior) of their year and region.
CREDIT
A Credit Certificate is awarded to a student who has not received a higher award but is in the top 50% (60% for Senior) of their year and region.
PROFICIENCY IN MATHEMATICS SKILLS AND PROBLEM SOLVING
This Certificate is awarded to a student who has not received a Credit (or higher) certificate, but who has nonetheless received a satisfactory score indicating satisfactory competence at mathematics and problem solving with mathematics. Students who score 32 points are guaranteed at least this certificate, but this might be lower if the exam has been deemed to be more difficult in their year level. The score as determined in Australia becomes the benchmark score for awarding this certificate in other countries, and is typically a recognition that they have reached a satisfactory level of mathematical and problem solving skills by Australian standards.
This certificate is awarded for the first time in 2008.
PARTICIPATION
A Participation Certificate is awarded to a student who has not received a higher award but has participated in the AMC.
PRUDENCE AWARD
This will be awarded to the student in each school with the highest number of consecutive correct responses starting from question 1. If there is a tie more than one such prize can be awarded in a school.
In order for the prize to be awarded the school needs to have an entry of at least 50 students and the winner must have at least the first five answers correct.
Note, this replaces the former Special Achievement Award.
MEDAL
Each year the AMF Committee awards a number of medals to students at Secondary levels. The number allocated is about 1 for every 10,000 students entered. These are awarded on the judgement of the committee to students who are outstanding within their region (Australian State or Territory or other country), within their year group and internationally. Within a region and year group is generally interpreted as meaning that no more than three such students can be deemed to be outstanding. If it is necessary to distinguish ties, including perfect scores, a second round of the competition shall be held, generally within six weeks of the main round. A prize, whose value will depend on such issues as sponsorship, is normally awarded to a medallist.
Source
(with his awesome array of prize money of 50 dollars in the past 4 years) 
