G4/AS1 Fifth Edition: NZ Ventilation Changes Explained

The transition to the Fifth Edition of G4/AS1 is now complete. Here's a quick overview of what has changed and what it means for mechanical contractors and building specifiers.

New Zealand's updated G4/AS1 Acceptable Solution for ventilation came into effect on 28 July 2025, with a 12-month transition period ending on 31 July 2026.

G4/AS1 provides a single recognised pathway for demonstrating compliance with Clause G4 (Ventilation) of the New Zealand Building Code.

The good news for the industry is that the Fifth Edition doesn't introduce major changes to ventilation performance requirements. Instead, the update is largely focused on improving clarity, consistency and the way relevant standards are referenced.

So, what has changed?

There are three main areas worth noting:

Clearer documentation: Updated terminology, headings and paragraph numbering make the document easier to navigate. Existing specifications may need their references checked.
Building Product Specifications (BPS): G4/AS1 now references MBIE's Building Product Specifications for relevant air-handling systems, bringing applicable product standards into a more centralised framework.
Updated references: Some standards references and definitions have been revised or relocated, but the underlying ventilation performance requirements remain largely unchanged.

What does this mean for mechanical contractors and specifiers?

With the transition period complete, new building consent applications using G4/AS1 should reference the Fifth Edition. Projects consented under earlier requirements may continue under their existing approvals.

For mechanical contractors and specifiers, the practical implications are straightforward:

Check that project specifications reference the appropriate current compliance documents.
Confirm which product and construction standards apply to the system being specified.
Ensure manufacturers receive clear requirements for materials, dimensions, pressure classifications and ductwork construction.

It's also worth remembering that G4/AS1 addresses ventilation compliance, while standards such as AS 4254 and SMACNA address aspects of ductwork construction. Meeting a ductwork manufacturing standard doesn't, by itself, establish compliance for the complete ventilation system.

Keeping specifications current

At ARLO Express Sheetmetals, we manufacture ductwork systems for commercial and industrial projects across New Zealand and Australia.

While ventilation system design and Building Code compliance sit with the relevant project professionals, accurate manufacturing depends on clear specifications and an understanding of the applicable construction standards.

The updated G4/AS1 framework is a useful reminder that good compliance outcomes depend on everyone in the supply chain understanding their responsibilities and working from clear, current information.

Because better buildings aren't simply about meeting the required standards. They're about having the processes, knowledge and relationships in place to deliver them consistently.

Further information: MBIE — G4/AS1 Fifth Edition