Finance Minister Bill English said he would consider taxing ACC's investments in order to bring it into line with the New Zealand Superannuation Fund.
The Super Fund, which is worth $29 billion, is the biggest single taxpayer in the country, pouring about $1 billion a year into the Government's coffers.
But since the Government stopped putting money into the Fund, when the global financial crisis hit in 2008, it has been paying out more money than it receives. Critics said that was limiting its ability to pay for future superannuation costs.
Mr English said taxation improved the Fund's performance.
"Because they've had to think, quite successfully, about the inherent risk-return trade off in their investment strategy, rather than make tax-advantaged investments where government can change the rules on them."
Mr English said taxing ACC was something the Government should think about.
"Ideally they should all be competing on a level playing field."
The current combined value of ACC's investment portfolios is about $31.5 billion.