Prime Minister Jacinda Ardern says she could not get the support of NZ First on a capital gains tax, but that the government will now find other ways to achieve greater fairness in the system.

Photo: RNZ / Ana Tovey

After pushing for the tax for three election campaigns, Ms Ardern told Morning Report she still believed in a capital gains tax but that it was now time to look at other options.

"I've been campaigning since the 2011 election but of course before that when we first proposed the capital gains tax ... we've tried different variations."

Ms Ardern on 17 April abandoned any plans to implement the tax - not just under this government, but for as long as she's in charge.

She said she fought hard for it and made the case but that it failed to pull through.

"Here we were finally in government, we had political parties that represented the majority of New Zealanders and I still couldn't get it across the line.

"I just couldn't get the numbers, and it wasn't for lack of trying."

Ms Ardern said her focus in the 2017 election campaign was to get the numbers for it.

"So I've done the public facing arguments on it ... my job then became getting the votes in Parliament."

Ms Ardern said she negotiated with party leaders directly but could not convince NZ First leader Winston Peters.

"No, I could not get the support of NZ First."

Mr Peters previously said he couldn't support the government introducing a capital gains tax because he saw no compelling evidence it would improve equality.

"I'd never seen anywhere in the world where the stated purpose for which it was introduced ended up being successful," he said.

"We decided there are better ways in which we might reform our taxes to give people a fairer go ... where the gap between the rich and the poor can be closed. This is not the pathway."

Ms Ardern said she just had to accept that in this term of office she did not have the numbers.

However, she said the government would continue to work on inequality.

"We can find, and will find greater ways to achieve fairness in our system," she said.

"There's a range of options but I need to go and develop that plan for 2020, because as I say we've just come off the back of a debate we've been having since 2011."

Ms Ardern has ruled out raising the top tax rate this term but said the Welfare Expert Advisory Group would be coming out very shortly with some of their recommendations.