The Labour and Green parties will be putting up a united front this morning as they seek to reassure the voting public of their economic know-how ahead of the September election.

The two parties are together releasing a set of economic bullet points called "Budget Responsibility Rules" this morning at a breakfast attended by bankers, economists, academics, and NGOs.

The rules include commitments to:
- Run surpluses every year, barring economic shocks or natural disasters.
- Reduce net debt to 20% of GDP before 2022.
- Prioritise investments in long-term challenges, such as superannuation, infrastructure and climate change.
- Keep Crown spending to around 30% of GDP.
- A progressive, fair tax system.

Green Party co-leader James Shaw said the plan establishes shared principles, and won't contain any numbers as it's not a tax package, policy, or an alternative budget.

"What this set of shared principles shows is that whatever view we have, whether we agree or disagree on any particulars, we are actually united when it comes to the principles on which we want to govern the economy," Shaw said.

Labour Party leader Andrew Little said any policy differences - such as a capital gains tax - will be worked out after the election.