The answer depends on your cashflow situation. Many businesses lease vehicles to avoid an upfront capital payment and therefore protecting cash for more direct expenses related to business growth but there are now some good deals for vehicles to purchase outright with delayed payment plans. I suggest you do a cash flow and cost comparison depending on how many vehicles, how long you intend to keep them for, the tax treatment etc. There is also a difference between finance leases (where you paying a hire purchase type of arrangement) and operating leases (maintenance, WOF, registration etc paid). I understand (please verify with your accountant), that lease payments are completely tax deductible as a business expense for all vehicles that are used ONLY for business (as opposed to vehicles that really are mostly private use which are treated as a perk). For company owned vehicles, the depreciation on the business use portion is tax deductible.
I also suggest you research the cost of personal use of the vehicles if there is any (technically the trip to get someone to and from work each day is a personal trip and therefore considered personal use). This is regardless of whether the vehicle is leased or purchased outright and will impact on Fringe Benefit tax payable and the amount of the expense that is tax deductible. It is a good idea to have a written company policy on this so that it is very clear to everyone, including the IRD. If the vehicles are parked up at work during non-work hours, then generally no FBT is payable.