Unions outspending Liberals on ads: Harvey

West Australia's deputy premier says the government is finding it hard to "cut through" to the public because it's being heavily outspent by the opposition and unions on political advertising.

Liza Harvey said unions were spending millions on ads and warned they would want something in return from opposition leader Mark McGowan if elected premier on March 11.

"Its very hard to cut through when you're being outspent so dramatically by the union movement," Ms Harvey said on Friday.

"The unions are out in full force, they are spending millions of dollars to put Mark McGowan in as a premier and if Mark McGowan is elected as premier, he will be paying back that expenditure."

The Labor Party has been contacted for comment.