So, I've been looking for details of exactly how a replica firearm must be stored in NSW.

Section 41 of the firearms act for a category H firearm.

FIREARMS ACT 1996 - SECT 41
Category C, D and H licence requirements
41 Category C, D and H licence requirements


(1) The holder of a category C, category D or category H licence must comply with the following requirements in respect of any firearm to which the licence applies:
(a) when any such firearm is not actually being used or carried, it must be stored in a locked steel safe of a type approved by the Commissioner and that cannot be easily penetrated,

(b) such a safe must be bolted to the structure of the premises where the firearm is authorised to be kept,

(c) any ammunition for the firearm must be stored in a locked container of a type approved by the Commissioner and that is kept separate from the safe containing any such firearm,

(d) such other requirements relating to security and safe storage as may be prescribed by the regulations.

Maximum penalty: 50 penalty units or imprisonment for 2 years, or both.

(2) A licensee does not have to comply with the requirements of this section if the licensee satisfies the Commissioner that the licensee has provided alternative arrangements for the storage of firearms in the licensee’s possession that are of a standard not less than the requirements set out in this section.
This makes it all too hard if you ask me.

Gok, I don't know where you get your plastic box with a lock rule from but I'd like to see the regs you're complying with and how I get the officer approving my application to use those regs instead of the NSW firearms act above.