National holiday road toll rises to 18

Seven people have died on South Australia's roads within 48 hours, taking Australia's festive season toll to 18.

A 13-year-old boy from Aldinga Beach died after the sedan he was travelling in and a van collided just before midday on Tuesday near Port Lincoln on the Yorke Peninsula.

The sedan's female driver was airlifted to hospital in a serious condition.

An 11-month-old girl and a six-year-old girl were also taken to hospital with non-life threatening injuries, as were the driver of the van and his two passengers.

Earlier on Tuesday, a two-year-old boy from Ballarat died in hospital after the four-wheel drive he was in veered off the road and crashed into an embankment at Clay Wells.

The 24-year-old driver and her 40-year-old female passenger sustained minor injuries.

In a separate accident, a man died when he was struck in a hit-and-run near a service station at Woodville North early on Tuesday.

Police said the man had demanded cash from two teenage girls while armed with a metal pole shortly before the crash.

Two motorcyclists were killed in separate incidents on the NSW south coast.

Emergency services discovered a male rider dead, and his female passenger with a broken arm, when they arrived at the southern end of Bellambi Beach on Monday night.

It's believed the rider may have fallen down a steep embankment.

Earlier, a 50-year-old man died after his motorbike and a car collided at Basin View, near Nowra.

NATIONAL CHRISTMAS/NEW YEAR ROAD TOLL AS OF 3.30PM AEDT, DECEMBER 30

Fatalities:

* NSW - 4

* VICTORIA - 2

* QUEENSLAND - 2

* ACT - 0

* WA - 2

* SA - 7

* NT - 0

* TASMANIA - 1

TOTAL: 18