Former PM Sir John Key arrives at National Party base

Former Prime Minister Sir John Key says he always thought Bill English would be "a strong campaigner" and called him the "heart and soul of the policy part of the party".

Sir John is at the National Party base tonight as the final results roll in.

"I always though Bill would be a very strong campaigner," he said tonight. "He has really been the heart of soul of the policy part of the party."

"Elections are won and last on the economy, and I think the country has shown tonight that they trust Bill with the economy. They trust National. Realistically I always thought it was possible, and he really stepped up on the campaign."

He told Newshub no one would have believed it was possible nine years ago for National to be looking at a fourth term and be getting a result around 46 per cent.

He felt the result proved he was right in thinking he could resign and pass the reigns to English.

Sir John resigned as Prime Minister in December last year, citing family reasons for leaving.

"This is the hardest decision I've ever made and I don't know what I'll do next," he said at the time.

Sir John said the job had required great sacrifices "from those who are dearest to me".

His wife Bronagh had endured "many lonely nights" and his children Stephie and Max had been put under "extraordinary levels of intrusion".