Interior design rates in New Zealand vary widely. You can expect to pay from a minimum of about $80 an hour for someone who isn't fully skilled, up to $400 an hour and beyond for a qualified, commercially recognised designer. The part that surprises people is that the more experienced designer often costs you less overall.
The bottom of that range buys you what I'd call a cushion fluffer: someone who can style a room but hasn't the training or the trade relationships to save you from expensive mistakes. The top of it buys judgement built over a career. Both charge by the hour, but you are not paying for the same thing.
Why an experienced designer can save you money
A qualified, commercially recognised designer should actually reduce the cost of your refurbishment, because you receive trade discounts through working with one. Beyond that, experience is efficiency. An experienced designer can do in an hour what a client might take weeks to work out, because we know exactly what not to waste time and money on to get maximum impact. The fee is real, but it is usually smaller than the money saved by not buying the wrong thing twice.
You are not paying for hours. You are paying for the years that make the hours count.
How designers charge
There are two common models. Some charge an hourly rate. Others charge a fee based on a percentage of the total spend. Either way, it should be able to be quantified before you engage anyone. Always ask for an estimate based on your brief, priced to the complexity of the service involved. A good designer will be happy to give you one, and it protects both of you.
The short answer
Budget somewhere between $80 and $400 an hour depending on who you engage, ask for a written estimate against your brief, and factor in the trade discounts and avoided mistakes on the other side of the ledger. Costed properly, good design pays for a good part of itself.