Ratings agency Standard and Poor's cut its outlook for the federal government's coveted triple-A rating on Thursday, citing uncertainty following the Saturday's election and high levels of external and household debt.

The outlook for Australia's credit rating has been changed to "negative" from "stable".

The decision does not mean the federal government has lost the triple-A rating, which marks it as an extremely safe borrower.

It is effectively a shot across the bow, warning Australia could face a downgrade if it does not do something to change the fiscal outlook.

That could deter investments and mean higher borrowing costs.

Standard and Poor's decision also has an impact on the ACT. A state or territory cannot have a higher credit rating, or a different outlook, than that given to the federal government.

That means the ACT will now have its credit rating outlook changed to negative.

Speaking from New Zealand, Chief Minister Andrew Barr said the result was frustrating, given the cause of the downgrade was completely out of the territory's control.

"What Standard and Poor's will do will move us to that same position, which is frustrating because it's not our doing, but it's the nature of the federation," he said.

"A negative watch is the ratings agencies sending a signal, so it's not a downgrade, it won't have any immediate impact for the territory.

"But if the Commonwealth don't fix their fiscal position in the short-to-medium term, if the Australian government's credit rating is lowered, every other state or territory governments' credit rating will be lowered."

Federally, Labor says the decision casts further doubt on the viability of the Coalition's budget projections and plans to cut the corporate tax rate.

Treasurer Scott Morrison has said the decision affirms the government's fiscal direction, showing Australia needed to stick to the current plan.

Mr Barr urged the incoming treasurer to move quickly to deal with the ratings warning.

"We have worked very, very hard to maintain triple-A and I'm confident we will still have triple-A," he said.

"But I would expect that the incoming treasurer federally will need to address this and that the parliament collectively, House of Representatives and Senate, all parties, because it is not in our country's interest to see the credit rating lost."

Mr Barr has repeatedly used the territory's triple-A rating as a mark of the success of economic reform, including taxation reforms.

It was also used in this year's budget to show that spending on light rail, a major area of attack from the opposition, had not impacted on the government's credit rating.

The impact of the ACT government losing its triple-A rating would be negligible, Mr Barr said.

When a credit rating is downgraded it can, in some cases, result in increases to interest rates on government debt, reflecting the higher risk of lending.

But Mr Barr said much of the territory's borrowings were already locked in at low rates.

He said the ACT had borrowed at a time that was best suited for big capital investments.