On Monday, New Zealand's government said it would consider joining any Australian legal case to force Japan to halt whaling.
"New Zealand is opposed to whaling in the Southern Ocean. We've made that quite clear. We've also made it clear our preference is to try and find and diplomatic solution," John Key, New Zealand's prime minister, told local media.
"If the diplomatic solution fails and the only option available is a court action, at that time we will consider whether we'll join Australia."
Japan's foreign minister, Katsuya Okada, who held talks with Rudd in Australia at the weekend, described the threat of legal action as "unfortunate".
The Japanese hunt off Antarctica kills hundreds of whales each year, drawing fierce criticism from Australia and New Zealand, both of whom have long been staunch opponents of whaling.
Commercial whaling has been banned worldwide since 1986 but Japan continues its annual hunts justifying them as "scientific research" which is allowed under a loophole permitted by the IWC.
Japan does not hide the fact that the whale meat is later sold commercially in shops and restaurants but says the money is used to cover research costs.
It also argues that whaling has been part of Japanese culture for centuries.