The sudden death of Green Party co-leader Rod Donald yesterday has left his party in shock and brought tributes to the vigorous MP from politicians across the spectrum.

The 48-year-old father of three was due to be sworn in to the new Parliament today for his fourth term.

He died at his Christchurch home of a suspected heart attack. The cause will be established by an autopsy.

His death is a shattering blow to Green MPs and staff, many of whom were in tears yesterday.

Co-leader Jeanette Fitzsimons said Mr Donald's greatest legacy was as a driving force in the campaign for MMP.

"I don't think we would have had proportional representation in Parliament without him."

Ms Fitzsimons struggled to keep her composure at a news conference yesterday.

"Rod is the last person that you would expect to die suddenly like this. He was young, he was fit, he didn't drink or smoke. He ran, he swam, he was full of life and energy. It has been a huge shock to all of us.

"He was my political other half and we were complementary. Our strengths were different, our weaknesses were different and I shall miss him enormously."

The Prime Minister led tributes from many political leaders and other organisations, saying she was "deeply saddened".

Helen Clark said she had known Mr Donald since he entered Parliament and had worked with him for the past six years, during which the Greens' support and goodwill had been indispensable to the Government.

"Rod Donald had remarkable energy and great zest for life. It is a cruel fate indeed which strikes down someone with so much to give in the prime of his life. He will be greatly missed."

National leader Don Brash said that despite disagreeing on some policies, he had always admired Mr Donald as a "hugely principled, honest and capable man, with a passion and a drive to represent his beliefs and speak his mind".

Mr Donald leaves behind partner Nicola Shirlaw and their three daughters, Holly, Emma and Zoe.

Ms Shirlaw told TVNZ that Mr Donald adored his children and his death was hard to take in.

"At the moment it's just too surreal because Rod often wasn't here and we just feel like he's not here, he's in Wellington or something."

Green MPs will get together with parliamentary staff this morning before the swearing-in ceremony in the afternoon, which is likely to be particularly poignant.

Ms Fitzsimons said she might talk to Clerk of the House David McGee today about how Mr Donald's death would be marked by Parliament.

She was inclined to leave the possum fur-covered seat alongside her in Parliament empty for a while.

A new co-leader would have to be elected at a party annual meeting, which was yet to be arranged. The co-leader did not need to be a sitting MP, said Ms Fitzsimons.

"I don't think it's a thing we should rush into. It's quite important that when a leader is replaced after 10 years that it is done thoughtfully ... and I don't see any problem if there's a vacancy for a little while."

The Greens were disappointed after the September election when they sought ministerial roles in coalition with Labour. The Prime Minister bowed to vehement objections from New Zealand First and United Future and the Greens did not enter a formal coalition.

Mr Donald and Ms Fitzsimons resolved to make the most of the policies they got on to the Government's programme, including a Buy NZ Made campaign that Mr Donald would have led.

Ms Fitzsimons said Mr Donald had been a tireless campaigner for fair trade and protection of local jobs and had been looking forward to the Buy NZ Made campaign.

Green MPs have vowed to carry on the campaign.

The co-leaders have been in the job for 10 years, starting at the 1996 election under the Alliance banner. At the 1999 election, the Greens scraped in after going it alone.

Yesterday, Ms Fitzsimons spoke to former MP Nandor Tanczos, who as the next candidate on the Green list would be in line to re-enter Parliament.

The Herald could not contact Mr Tanczos last night.