In 2004 the Liberal Party made great use of the tag L-plate leader in relation to the Labor leader, Mark Latham. It was neat because of the conjunction of the L of the L plate and L at the start of Latham’s name.
The Age explained it at the time: The Federal Treasurer, Peter Costello, launched the campaign billboard which uses an L-plate to represent the first letter of Mark Latham's name and says "Latham and the economy, Good Luck".
This figurative use of L-plate to mean ‘rookie’ or ‘beginner’ with overtones of ‘incompetent’ may well be an Australianism.
There have been occasional instances of L-plate leader and L-plate politician in the intervening years, but the expression was revived again when the Environment Minister, Melissa Price, derided Anote Tong, the former president of Kiribate, by saying that he, and all his friends from the Pacific Islands, came to Canberra to get money. She offered to pull out her chequebook and write him a cheque.
This produced a response from another Liberal politician, Senator Fierravanti-Wells, who described Price as an Environment Minister on her L-plates. Since then Price has not been able to shake the title ‘L-plate Environment Minister’.