A5.10.1 provides provision for an Immigration Officer to not require a police certificate if the Immigration Officer is satisfied that such a certificate is not available or would be unduly difficult to obtain. I would guess that the onus is on you to prove this to INZ's satisfaction.
If you do satisfy INZ to this effect, you will likely be required to make and provide a statutory declaration in both English and your home language, that details your attempts to obtain a police certificate, states whether you and any accompanying family members have been convicted, or found guilty of, or charged with offences against the law of that country, or have not been charged with any offences against the law of that country and must be corroborated by other information confirming your character.
I suggest you go through the entirety of A5.10 -- it should answer your queries in this regard.