The jump from 4-berth to 6-berth isn't just two extra sleeping spots — it's a longer vehicle (typically 7.0-7.5 m versus 6.5-7.0 m), a higher fuel use (14-17 L/100km versus 12-15), and a different drive feel on NZ's mountain passes. For a family of four with primary-school-age kids, the 4-berth is almost always enough. For a family with teenagers, two couples travelling together, or three generations across grandparents-parents-kids, the 6-berth's separate Luton bed plus rear bed plus dinette conversion earns its rental premium.
Sleeping arrangements that matter
Most 6-berths sleep six only with the dinette converted nightly. Some sleep six with two permanent beds (rear + Luton) plus a permanent third bed (drop-down or bunk), which preserves the dinette as a lounge after bedtime. If you're three couples or six adults, verify the bed permanence layout before booking — the photos sometimes show the daytime configuration only.
Drive feel and route shape
6-berth motorhomes handle every standard NZ highway — SH1, SH6, SH8 — without issue, and they're allowed on the Milford Road. The trade-off is slower mountain passes (Crown Range, Haast, Lewis Pass), more careful low-speed manoeuvring in town centres (Akaroa, Russell, Picton), and tighter holiday-park sites in older parks. Most modern holiday parks have at least one super-site that fits a 6-berth comfortably.
Cost reality
A 6-berth in shoulder season runs NZ$280-420 per day versus NZ$200-310 for a comparable 4-berth. Over 14 nights that's NZ$1,100-1,500 more. Split between three couples or six travellers, that's NZ$180-250 each — usually cheaper than two 2-berths plus the awkwardness of caravanning two vehicles in convoy.