Nikkiglitzy is correct. Put more simply, if the government borrows domestically to fund a budget deficit, they're going to be taking loanable money away from other domestic borrowers. This is especially problematic in a country like Australia which has a really low savings rate because #treatyoself. Just like with any product, smaller supply = higher price, so the price (interest rate) of those loanable funds goes up. Some domestic borrowers will be 'crowded out' of the credit market because of the higher interest rate. Thanks for nothing Scomo.
It's more of an issue if foreign borrowing doesn't happen, but in reality because domestic borrowers can borrow from overseas the crowding out effect isn't as pronounced as it theoretically could be. Though as said above this isn't great for the CAD.