He confirmed supply for more than 100 shipping containers for upcoming summer festivals and is expecting more to depart at the end of the month.
McLaren Falls will receive 20 containers, Laneway Festival 12, Auckland City Limits 10 and five for Raggamuffin.
Shipping containers can be modified into anything from temporary accommodation, public walkways, architecturally designed offices, pop-up kiosks and cafes.
Laneway festival promoter Mark Kneebone said the portable buildings are "super handy".
"At Laneway we stack them up high on top of each other and put them behind the main stage in a horseshoe shape as dressing and dining rooms for the artists."
Kneebone said the containers fitted with windows and doors arrive four days before the festival and are taken away after the event.
Rolf Wolf was one of the main suppliers of temporary modified containers for Christchurch businesses after the earthquakes in 2011.
"Containers have really found their feet. They've come in to vogue and they're sexy and trendy and cool - everyone wants a container," said Millen.
"They are the ultimate recycle product as you can repurpose them easily and efficiently."