Households are expected to be slugged substantial cost of living increases in the West Australian government's second budget.
Water Minister Dave Kelly has already revealed water charges for the representative household will increase by 5.5 per cent in 2018/19, and softened the blow by saying that compared to six per cent set by the previous Liberal government under its pricing model.
Mr Kelly also unveiled higher charges for households that use a lot of water, saying the hikes aim to make guzzlers cut down their water use, which will defer the need to build a new $1 billion desalination plant.
But shadow treasurer Dean Nalder told reporters ahead of Thursday's budget the Water Corporation was making tidy profits and could pay off a desalination plant within 12 months.
"If you've got a utility that's over-recovering to this extent, what is the basis for increasing the charges six times the rate of inflation?"
He said the Barnett government, in its last four years, never ended up following through with around seven per cent increases flagged in the forward estimates because of tough economic conditions.
"We only ever increased them by 3.5 or four per cent."
Mr Nalder said the McGowan government should take into consideration factors such as stagnant wage growth and a big rise in applications for the Hardship Utility Grants Scheme last year.
He said the HUGS trend in recent months was hard to know because in December, the state government made utility customers with overdue accounts go on payment plans for at least 180 days before they could access the scheme.
Other already-revealed cost of living increases include rises in the Emergency Services Levy to create a rural fire service.
Metropolitan landowners will pay an additional $28 from July 1 while landowners in the regions will pay between $8 and $17 extra.
"We think it's inappropriate that the state government at this point in time is putting pressure on the households of Western Australia," Mr Nalder said.
Treasurer Ben Wyatt has already revealed the 2018/19 deficit has narrowed to about $900 million, down from the $915 million forecast in the mid-year budget review.
He also says the state remains on track for a surplus by 2020/21 but it will take a long time to pay off debt, which was forecast in December to peak in 2019/20 at $42.8 billion.
"They haven't tackled it," Mr Nalder said.
He said the McGowan government, which inherited almost $32 billion in debt when it swept to power last year, needed to slow down its rate of spending.