The strategic direction of the Defence Force is up for debate at an academic symposium in Wellington today.

A group of academics from Victoria and Massey Universities plan to zero in on the Government's Defence White Paper - committing $20 billion to the military over the next 15 years.

Centre for Strategic Studies Robert Ayson said the "big three" of the Defence Force's capabilities are top of the list of discussion topics.

He says the Airforce's heavy-lifter Hercules, maritime patrol plane, the Orion, and the Navy's ANZAC frigate, are all tipped for replacement.

"Even though we've been upgrading them, and are upgrading them at the moment, we are going to find that they'll end their serviceable period over the next dozen to 15 years. The Hercules really need looking at and replacing almost right away, the Orion next, and then the frigates."

But Ayson said the government is still to specify what exactly the replacements will need to be able to do.

"The government has said they are expecting a major addition review of our strategic airlift requirements in terms of South-Pacific and perhaps Antartic requirements over the next two years, so there's quite a bit of close look to happen."

It's important to have public debate about how the government invests in Defence, and what New Zealand's strategic options are, he said.