Cullen - whose previous Labour roles have included Deputy Prime Minister, Finance Minister and Attorney-General - said he personally supported euthanasia.
He said he was already being kept alive by medical science, courtesy of a pacemaker installed two years ago. However, if he lost his mental or physical functions he supported the right to choose to die.
"My own personal position is that if it was me, I don't want to get to the state where my mind deteriorated to the point where I might actually support the National Party. In other words, I've forgotten who I am and what I am."
Cullen said Robertson's "Future of Work" paper was a well-thought-out piece - but warned Robertson could expect to face a lot of demands for spending if Labour was elected to Government.
He said years of restraint and cutbacks by National had created pressure points in the public sector, such as in health.
"You've got all these issues starting to bubble up so the pressures are going to be very strong for increases in spending."
"No doubt many public-sector unions banging on doors looking for catch up pay increases after years of very, very little. But then, that is exactly what a Labour Government has to cope with coming into office almost every time. There's nothing new about that."
He said that added to that would be the demands of coalition partners.