Budget 2019 has set aside only $150 million for the Government's buy-back scheme for firearms that were recently made illegal.

That is in the middle of Treasury's previous estimate that the buy-back scheme will cost between $100 million and $200 million - but gun lobby groups have estimated it could cost more than $500 million.

Gun owners expressed outrage at the Government's gun law reforms following the March 15 terror attack, and have been waiting to see if the buy-back scheme will be fair.

A further $18 million has been put towards the police scheme, which is still being worked out, for collecting the semi-automatic military style firearms that are now illegal.

Police have already said it would prefer to collect the guns, rather than have people with firearms queuing up outside police stations.

Finance Minister Grant Robertson said more money could be found for the buy-back scheme, if needed.

Budget 2019 concentrated on child wellbeing and law and order announcements were not the focus.

But there was extra funding, including $25 million to beef up efforts to stop boat people getting to New Zealand shores, $50 million for spy agencies the NZSIS and GCSB, and $13.4 million for next year's cannabis referendum.

Australia has criticised New Zealand for being too reliant on Australia to stop boat people from trying to get to New Zealand, though the Government has repeatedly said that there are few, if any, concerted efforts from people-smugglers to undertake the perilous journey to New Zealand's shores.

Further funds for the justice sector include $212 million towards building 16 remand homes over four years to house 100 young people caught in the youth justice system, which from July 1 this year will include 17-year-olds for the first time.

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That package is part of a larger child wellbeing package, which also includes $26.7 million over four years for Oranga Tamariki's NGO partners to improve intensive intervention services.

That is aimed to help keep at-risk children at home, rather than entering or re-entering state care. This hopes to cover 150 families and 400 children in its first year.

As at March 31 this year, 6400 children were in state care.

Half a billion dollars over four years has also been set aside for new Oranga Tamariki national care standards that come into force on July 1 this year.

Part of that will fund 350 more frontline staff and social workers and 60 extra support staff for caregivers, and $70 million will go towards a special individual bag of items for those in care that provides books, laptops, toys, health products and sports gear.

The Government has already announced $153.7 million to help young people transition from care or youth justice services back into the community, a service that currently does not exist.