Finance Minister Grant Robertson is giving little away ahead of the all-important opening up of the Government's books tomorrow.

But he has warned that unveiling of New Zealand's GDP numbers later this week will "not be pretty reading".

The Treasury will tomorrow unveil its Pre-election Economic and Fiscal Update (Prefu) – something it required by law to do ahead of every New Zealand election.

It will reveal a number of important sets of economic data, such as the expected level of Government debt, expected tax revenue numbers as projected deficits going forward.

That debt level took centre stage during the Budget, when Treasury revealed Government debt would rise to $200 billion by 2024 – that's up from the just under $90 billion that was initially expected in 2020, before Covid-19 hit the economy.

Speaking at a virtual BusinessNZ debate this morning, Robertson said very little about Prefu – only that despite the fact that New Zealand's debt level will climb to record highs, "it still compares very well with other countries".

And it's that level of debt and the Treasury's projections which will be front and centre for National tomorrow.

The party has deliberately held off putting out its much-anticipated tax and revenue policy until it's had an opportunity to take a detailed look at Prefu.

Under the microscope of that policy will be National's commitments to bring debt down to 30 per cent of GDP "within a decade or so".

"The 'or so' is important," the party's finance spokesman Paul Goldsmith said, given the high levels of uncertainty at the moment.

This has been a particular area of focus for Labour too.

The party has put pressure on National to explain how it would achieve such an ambitious debt target without cutting key areas of public spending.

National has countered that criticism by saying it would reveal details after it's looked through Prefu.

In an economic note, ANZ economists say they are not expecting "wholesale changes" to what the Treasury will unveil tomorrow, adding it is likely to be broadly similar to the numbers in the Budget.

But when it comes to GDP numbers – which will be revealed by Statistics NZ on Thursday – ANZ has warned of a big contraction, predicting quarterly growth will be -12 per cent.

Although this is down on its previous -17.5 per cent forecast, the data will still highly likely reveal that New Zealand is in a recession – two quarters in a row of negative GDP growth.

Robertson said the drop won't make for "pretty reading" but he did say the economic contraction would be "one we can recover from".

Goldsmith broadly agreed but blamed a "very hard lockdown" in March and April for what is expected to be one of the worst economic quarters on record.