Prime minister John Key is defending his overseas travel schedule - saying his more than 40 days out of the country give him time to think.

Key leaves this morning for the East Asia Summit in Phnom Penh, Cambodia and Myanmar.

Already this year, he has visited Korea, the US, Indonesia, Singapore Australia, Britain, Germany and Belgium, spending around 41 days outside of New Zealand.

''I do enjoy it. It gives you time to think when you take a step back. As prime minister, it's not just the quantity of decisions you make but the quality of them.

''I travel everyday - I'm constantly travelling. Yeah, it's a bit further afield but you get a perspective on what's happening internationally. And New Zealand is a very open economy so it's actually important to what we do.''

In Cambodia, Key will rub shoulders with re-elected US President Barack Obama, Australian Prime Minister Julia Gillard and outgoing Chinese Premier Wen Jiabao.

On Wednesday, he will also visit the Extraordinary Chambers in the Courts of Cambodia (Khmer Rouge Tribunal) where former governor-general Dame Sylvia Cartwright sits with four other judges.

''She is undertaking a very challenging process. We are trying to physically demonstrate our support for her and the job in hand,'' Key said. ''It's important that those that committed war crimes are held to account.''

Key will then fly to Yangon, the first visit of a New Zealand prime minister to Myanmar. The impoverished south east Asian nation has been diplomatically isolated for years, but recent political and economic reforms have prompted other world leaders, include Obama, to visit.

He will tour the glittering Buddhist Shwedagon Pagoda, and Taukkyan War Cemetery, where nine Kiwi soldiers killed in the Second World War, are buried.

In the capital Naypyitaw he will call on President Thein Sein and attend a state banquet and hold talks with celebrated opposition leader Aung San Suu Kyi.

Before returning to New Zealand on Friday, Key will also visit the impressive Parliament, noted for the often deserted 20-lane highway which leads to its entrance.

Key says the public appreciate the importance of his overseas travel.

''New Zealand is a small open economy that needs to make its way in the world. A lot are pro-forma...but the [visits] I would say were discretionary were LA - we didn't have to do that trip but we thought it was worthwhile [and] the trip to Australia - it's our biggest market.''

He added: ''It's not going to be any better next year. I don't think it's any less for any other prime minister.''

- © Fairfax NZ News