Yes, replacing a mortice lock is something a capable DIYer can tackle in most cases — it's not restricted work under NZ law, so no licence is required. That said, older NZ homes (think villas, bungalows, and state houses) often have non-standard door thicknesses, aged timber, and legacy lock sizes that can turn a straightforward swap into a tricky job. Read on to understand what's involved before you reach for the chisel.
What Is a Mortice Lock?
A mortice lock is fitted into a rectangular pocket (the mortice) cut into the edge of a door, rather than being surface-mounted. They're common in pre-1980s NZ homes and come in two main types:
- Sashlock – includes both a latch and a deadbolt, operated by a handle and key.
- Deadlock – key-operated bolt only, no latch. Often used as a secondary lock.
Older NZ homes frequently used imperial-sized mortice locks (British Standard sizes), which can be harder to match with modern metric replacements.
Tools and Materials You'll Need
- Tape measure and pencil
- Chisels (sharp — blunt chisels split old timber)
- Mallet
- Screwdrivers (flathead and Phillips)
- Drill with wood bits
- Replacement mortice lock and matching strike plate
- Sandpaper or a sharp plane (for fitting adjustments)
Step-by-Step: Replacing the Lock
1. Measure the Existing Lock Carefully
Before buying anything, remove the old lock and measure:
- Backset – distance from the door edge to the centre of the keyhole
- Case dimensions – height, width, and depth of the lock body
- Door thickness – older NZ doors can vary significantly
Take the old lock to your hardware store or locksmith supplier. Stores like Bunnings and Mitre 10 stock a range of mortice locks, but for heritage or non-standard sizes you may need a specialist locksmith supplier.
2. Remove the Old Lock
Unscrew the face plate on the door edge, then remove the handle spindle and rose plates on both faces of the door. The lock body should slide out of the mortice pocket. If it's painted in or swollen, work it free gently — avoid forcing it, as old timber can split.
3. Test-Fit the New Lock
Slide the new lock body into the existing mortice. Ideally it should fit snugly without modification. If the new lock is slightly smaller, you may need to pack the gap with timber. If it's larger, you'll need to carefully extend the mortice with a chisel — go slowly and work with the grain.
4. Mark and Fit the Face Plate
Hold the face plate flush with the door edge and mark around it with a pencil. Chisel a shallow recess (rebate) so the plate sits flush. Screw the lock body in place.
5. Fit Handles and Spindle
Thread the spindle through the lock, attach both handle roses, and check the latch operates smoothly. Adjust if needed.
6. Fit the Strike Plate
Close the door and mark where the bolt and latch meet the frame. Chisel a mortice in the frame for the bolt, and a rebate for the strike plate. The strike plate should sit flush with the frame. Test the lock fully — both latch and deadbolt should engage cleanly without forcing.
Challenges Specific to Older NZ Homes
- Swollen or warped timber – NZ's humid climate (especially in Auckland and coastal areas) causes timber doors to move seasonally. A lock that fits in summer may bind in winter.
- Lead paint – Homes built before 1980 may have lead paint on door edges. Sand carefully and wear a P2 dust mask. See WorkSafe NZ for guidance on working safely with lead paint.
- Heritage doors – Kauri and rimu doors are beautiful but can be brittle. Take extra care with chiselling.
- Non-standard sizing – Imperial-sized locks (e.g., 3-lever British Standard) may not have a direct modern equivalent. A locksmith can often source or adapt suitable replacements.
Does This Require a Building Consent?
Replacing a like-for-like mortice lock is generally exempt from building consent under Schedule 1 of the Building Act 2004 — it's routine maintenance. However, if you're modifying the door or frame significantly, or the work is on a heritage-listed property, check with your local council first. MBIE Building Performance has useful guidance on what work requires consent.
Security Considerations
If you're upgrading for security, look for locks that meet NZS 8611 (the NZ standard for locks and latches) or an equivalent recognised standard. A 5-lever deadlock provides significantly better security than a basic 2-lever lock. For rental properties, landlords should also be aware of Healthy Homes requirements around security and weathertightness.
When to Call a Professional
Call a qualified locksmith if:
- The mortice pocket needs significant enlargement and you're not confident with chisels
- The door or frame is damaged and needs repair before a new lock can be fitted
- You're working on a heritage or character home where mistakes could be costly
- You want a high-security lock system with master-keying
- The door is badly warped and needs planing or rehinging — this may involve a joiner as well
Locksmith work is not restricted work under NZ law, but a professional will have the right tools, experience with older NZ door types, and access to a wider range of lock sizes. You can find qualified local locksmiths on tradedirectory.co.nz.
