The McGowan Government has been hit by an embarrassing leak just 24 hours after its first budget, with an internal Labor document on it sent straight to the party's political enemies.

Talking points prepared by the Government, detailing how the Premier's office wants MPs to spin the budget, were leaked to the Liberal Party within hours of being produced.

While the production of talking points by a leader's office is not unusual, the leaking of them to political opponents is extremely rare.

Opposition Leader Mike Nahan said the document was sent straight to him, seizing on it as evidence of disunity within Labor's ranks just six months after the Government was elected.

A spokesman for the Premier did not dispute the legitimacy of the document.

According to the document, the Premier's office suggested MPs defend the Government's broken promise not to hike taxes by saying they had been "forced" into it.

"Everyone across the WA community needs to share the burden to help pay for the Liberals and Nationals' out-of-control spending, from the public sector to the state's largest companies," the document stated.

"Tough decisions have been made to strike the right balance to repair the budget and help boost the WA economy.

"The Premier has apologised for being forced to break his promise and has made sure that struggling families and small business have been protected."

Bank tax 'would raise $950m'

The talking points also reveal a South Australian-style bank tax remains firmly on the Government's agenda, despite not being included in the budget.

"If WA was to introduce a bank tax like the one in South Australia, it is expected it would provide about $950 million in additional revenue over the forward estimates," it stated.

And the document encourages MPs to downplay the impact of the payroll tax increase, while indicating it wanted to portray the big miners as a target of the change.

"Around one-fifth of the additional revenue generated from the measure is expected to be paid by the larger iron ore producers," it stated.

"This is a reasonable decision we have had to make to fix the budget mess left behind by the Liberal National Government."

Dr Nahan said it was severely embarrassing for the Government that there were already efforts from within Labor to undermine the budget.

"That they leaked to me indicates there is a great deal of dissent within the McGowan Government," he said.

The Premier's office would not specifically comment on the leak, but defended the production of talking points to be sent to MPs, something which previous governments also did routinely.

"Unlike the previous government that spent hundreds of thousands of taxpayers' dollars on consultants to develop budget communication plans, there were no costs associated with the 2017-18 budget, with everything being produced in-house," a spokesman said.