Legal basis for direct democracy at the national level

Government-initiated referenda conducted by postal vote are governed by the Referenda (Postal Voting) Act 2000. 

Citizen-initiated referenda conducted by postal vote are governed by the Referenda (Postal Voting) Act 2000 and the Citizens Initiated Referenda Act 1993.

Government-initiated referenda conducted other than by postal vote may be held, but must be authorised and governed by specific, ad-hoc enabling legislation. 

Section 24 and Section 24A of the Citizens Initiated Referenda Act provide for the application of Electoral Act 1993, excluding the sections stated in articles 24(5) and 24A(3), to referendums.

Referenda (Postal Voting) Act 2000;

(Enacted on 14 November 2000)

http://www.legislation.govt.nz/act/public/2000/0048/latest/DLM73881.html?search=ts_act%40bill%40regulation%40deemedreg_postal_resel_25_h&p=1

Citizens Initiated Referenda Act 1993

( Enacted on 28 September 1993)

http://www.legislation.govt.nz/act/public/1993/0101/latest/DLM317193.html?search=ts_act%40bill%40regulation%40deemedreg_citizens_resel_25_h&p=1

Electoral Act 1993

(Enacted in 17August 1993, last amended in 1 July 2012)

http://www.legislation.govt.nz/act/public/1993/0087/latest/DLM307519.html

Electoral Regulations 1996

(6 May 1996)

http://www.legislation.govt.nz/regulation/public/1996/0093/latest/DLM209678.html

For an example of specificenabling legislation providing for Ad hoc government-initiated referenda,

see:

Electoral Referendum Act 2010

(Enacted on 20 December 2010)

http://www.legislation.govt.nz/act/public/2010/0139/latest/DLM2833513.html?search=ts_act%40bill%40regulation%40deemedreg_referendum_resel_25_h&p=1

New Zealand, inheriting the UK/Westminstermodel, has no codified Constitution providing for direct democracy mechanisms.