Australia's defence forces could be grounded within weeks of an attack due to a desperate lack of fuel reserves, an incoming Liberal senator and retired senior military officer has warned.

Key points: Australia one of few countries without a government-mandated fuel reserve

Australia one of few countries without a government-mandated fuel reserve Jim Molan warns Australia can't rely on US Government to protect it

Jim Molan warns Australia can't rely on US Government to protect it The retired senior officer says Australia may need to further increase military spending

Jim Molan was the chief of operations for coalition forces in Iraq and will enter Federal Parliament next month, replacing former deputy Nationals leader Fiona Nash.

He has issued a stark warning about Australia's readiness for war, saying the armed forces could be ineffective within 19 days if current stockpiles of petrol, diesel and aviation fuel run dry.

"We are almost unique throughout the world in that we don't have a government-mandated strategic reserve of fuel," Senator Molan said.

"You can have all the fantastic equipment this Government is building and buying, but if you haven't got fuel and you haven't got missiles for them, then we've got a discontinuity at the centre of our strategy."

"There are things that we can probably never build in this country, such as the Joint Strike Fighter and the most advanced missiles," he continued.

"But we should guarantee their delivery to Australia — which you can rarely do — or we should have them in warehouses.

"Unless we have a plan to get them when we need them … then I, as an ex-military commander, wouldn't want to cross the start line in doing something militarily unless I had those warehouses behind me."

The 2016 Defence White Paper warned Australia's dependency on fuel imports was a risk given tensions in the South China Sea, which is a major shipping route.

Senator Molan said governments needed to better communicate the risks of not investing in defence.

Last year, United States President Donald Trump criticised NATO members and allies who failed to commit at least 2 per cent of their gross domestic product to defence spending.

While Australia has committed to that target — announcing $30 billion in additional spending in the next decade in the 2016 White Paper — Senator Molan said that might not be enough.

"If American power is relatively in decline, is 2 per cent (of GDP) enough?" he said.

"You can't just hit 2 per cent and achieve military perfection, you've got to stay at that level of expenditure."

Molan: US military assistance not guaranteed

Liberal senator Jim Molan says Australia's should not assume the US military will always protect it. ( ABC News )

Senator Molan has also warned that military support from the US is not guaranteed and the Federal Government needs to be more prepared.

"Australia should be thinking about the level of defence expenditure that we are prepared to commit ourselves if America was the centre pole of our defence policy and now may not be as strong as it once was," Senator Molan said.

Senator Molan said US Defence Secretary James Mattis has raised concerns about the US military's readiness for war.

"That should be ringing bells all over the world," Senator Molan said.

"John McCain, who is the chairman of the Senate Armed Services Committee, says exactly the same thing: America's military advantage is eroding particularly when it comes to Russia and China."

Senator Molan said the relationship with the US Government remained strong even if military capability was declining.

"President Trump when he was campaigning spoke about the amount of money he was going to put into defence and what he was going to create," Senator Molan said.

"He hasn't been able to do that."