[b&k]

I'm thinking of going to NZ on a tourist visa then finding work. In order to get PR as soon as possible, I was thinking of getting my police checks and medicals done before I go.

Does anyone know how for long these remain valid? Are there any other advantages/disadvantages to doing this?

[Sam B]

Not very long. It's 3 months for medicals I think, and maybe 6 months for police checks.

[Oregonkiwi]

yes, 3 & 6 months:
For each person included in your application you will also need to provide:
two recent passport-size photographs
a valid passport or certificate of identity
a completed Medical and X-ray Certificate – less than 3 months old
a police certificate less than 6 months old for each applicant aged 17-years and over for each country of citizenship and each country in which they have lived for 12 months or more in the last 10 years.
a full birth certificate.
http://www.immigration.govt.nz/migrant/stream/work/workandlivepermanently/howdoiapply/residence/thesteps/default.htm