The 11th-hour funding commitment was made in Bomaderry during a flying visit by Mr McCormack to announce the long-awaited Milton Ulladulla bypass would receive $400 million of the $500 million in federal funding committed in the May budget.

During a swipe at Labor's preference for 50-50 commonwealth-state funding arrangements for road improvements, Mr McCormack said the Coalition would come on board with an 80-20 split.

"Well, the Coalition - the Liberals and Nationals - we do 80-20," he said.

"The Commonwealth pays 80 per cent of the bill and the state comes on board with the other 20 per cent. That makes sure we get things done."

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Flanked by Nationals and Liberal candidates for Gilmore Katrina Hodgkinson and Warren Mundine and NSW Roads Minister Andrew Constance and South Coast MP Shelley Hancock, Mr McCormack was asked whether his comments were a firm commitment.

"Absolutely. Andrew [Constance] and I have always talked 80-20 splits - 80 Commonwealth, 20 state. That's how we roll with regional road funding. That's our commitment."

The Deputy PM commended Ms Hodgkinson for her advocacy for highway funding and Australian Community Media newspapers on the South Coast for their FIX IT NOW campaign.

Labor candidate for Gilmore Fiona Phillips dismissed Mr McCormack's commitment.

"The reality is the Liberals and Nationals are only committing $50 million to the Princes Highway over the next four years and Labor is committing $150 million," she said.

"The Liberals and Nationals are playing catch-up. As a local I know this is a top priority and Labor announced it as a priority project when we first committed $500 million to the Princes Highway."

Mrs Phillips said she would not enter into "a pathetic debate about percentages".