If an employer underpays you, you can always look for another
job that pays higher - provided you have the skills to justify the
pay. At the end of the day, it is your skill level that determines
your pay.
If you cannot find another job that pays higher, you better go
for retraining.
What is the minimum wage for S pass holders? The minimum wage
for S pass holders is to stop employers from hiring too many of
them following complaints of too many foreigners.
The purpose of minimum wage is always, since the time it first
started in Australia and NZ, to prevent employment. Let me give you
a simple illustration to convince you, since you do not appear
convinced.
Since minimum wage started in Australia/NZ, I will give you an
Aussie illustration. 
It is sometime in the 1890s. Mr White is an experienced sheep
shearer. Mr Wong is just fresh of the boat from China and
inexperienced in Aussie jobs like sheep shearing. Also the job
needs strength and White is bigger and stronger than Wong. Mr Rich
is a wealthy owner of a sheep station (Aussie slang for ranch or
farm). He has 400 sheep that needs shearing.
Mr White can shear 12 sheep in one hour. Mr Wong can only shear
4 sheep in one hour. Mr Rich is prepared to pay Mr White $3 per
hour and Mr Wong $1 per hour. Mr Wong does not mind because he
knows that Mr White can do the job three times faster than he
can.
In 25 hours, White sheared 25 x 12 = 300 sheep. He was paid $25
x 3 = $75.
In 25 hours, Wong sheared 25 x 4 = 100 sheep. He was paid
$25 x 1 = $25.
Therefore, Rich paid a total of $100.
Now watch what happens when they imposed a minimum wage of say
$2 per hour.
In 25 hours, White still sheared 300 sheep and he was paid
$75.
In 25 hours, Wong still sheared 100 sheep but now he would have
been paid $50.
This would have cost Rich a total of $125.
But Rich did not become rich because he was stupid. So he
decided to fire Wong and let White do all the sheep by himself.
Since White can shear 12 sheep in one hour, he finished the job
in 400/12 = 33.33 hours. He was paid 33.33 x $3 = $100.
Result? Rich spent the same as before minimum wage. White gets
more pay while Wong is jobless. Wong also lost the opportunity to
gain experience and in time would have qualified him a higher
salary.
This is a simple illustration of how minimum wage causes
unemployment among the low skilled. It does not help the poor. It
makes things worse for them.
