So-called “tweaks” to Jim Chalmers’ superannuation plan will punch a multibillion-dollar black hole in the Budget that economists warn could be filled with an increase to GST or changes to tax breaks on investment properties.

The Treasurer on Monday walked back two key parts of the superannuation changes, some 30 months after first proposing them.

The changes mean that the $3 million threshold for the high tax will be indexed and unrealised or on-paper capital gains won’t be taxed.

In its first full year of operation, in 2028-29, the Budget bottom line will only be $500,000 worse off, compared with the original plan.

But that gap blows out to almost $1 billion once a new scheme to boost super top-ups for people on low incomes is taken into account.

Over four years, however, the Budget

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