If we have the law of karma, why do we need other laws, or do we? Since we have the law of karma then there should be no need for laws or prisons? The law of karma will issue out punishment automatically. So if you murder someone there will be equal consequences regardless. Is this correct?
Even in a world where the law of karma is real and in effect, one needs laws and punishment for a few reasons.

Karma doesn't "issue", and its effects are not "punishment"; in Buddhist view it's a blind process of cause and effect and the effects of karma are not to be known or speculated about. This is one of the four imponderables:


  § 22.    "These four imponderables are not to be speculated about.
  Whoever speculates about them would go mad &amp; experience vexation.
  Which four? The Buddha-range of the Buddhas [i.e., the range of powers
  a Buddha develops as a result of becoming a Buddha]... The jhana-range
  of one absorbed in jhana [i.e., the range of powers that one may
  obtain while absorbed in jhana]... The results of kamma... Speculation
  about [the first moment, purpose, etc., of] the cosmos is an
  imponderable that is not to be speculated about. Whoever speculates
  about these things would go mad &amp; experience vexation."


http://www.accesstoinsight.org/lib/study/kamma.html#causes

It's not known when the effect of karma comes to fruition, in what way, or if they come to fruition at all. If karma comes to fruition, one doesn't necessary realize: "This is because of my bad karma, I should change my ways."

Therefore, the concept of karma doesn't:


repel those who want to commit crimes
remove those who commit crimes (temporarily) from society
rehabilitate those who have been found guilty


Which, in an ideal justice system, laws and prisons do.