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Before becoming Prime Minister, Scott Morrison was fond of declaring the government was tightening its belt to get the budget back into surplus.

Fastening his own belt buckle was another matter, with the 50-year-old married father-of-two sporting a protruding belly when he triumphed in the Liberal Party leadership ballot just two months ago.

ScoMo, as he is affectionately known, has dramatically tightened his waistline during the past three weeks, amazing some of Australia's leading dietitians.

Scott Morrison (pictured at the Bathurst 1000 in early October) was fond of declaring the government was tightening its belt to get the budget back into surplus

The Prime Minister's belly was visible under his white shirt at the end of August when he met Indonesian President Joko Widodo (right) during his first week as Australia's 30th prime minister

A weight loss company boss, however, has pleaded with the Prime Minister to avoid the Atkins diet, under which carbohydrates are shunned, after he was this month photographed eating two sausages without any bread at Parliament House in Canberra.

His recent visible weight loss has been dramatic. 

His belly was visible under his white shirt at the end of August when he met Indonesian President Joko Widodo at Bogor during his first week as Australia's 30th prime minister.

In the first week of October, his stomach protruded under a button-up shirt as he greeted Holden V8 Supercars champion Craig Lowndes at his last-ever Bathurst 1000 in the New South Wales central west.

This week, he appeared to be a lot fitter as he visited a gelato shop in Canberra.

Trent Watson, an accredited practising dietitian from Newcastle who represents the Dietitians Association of Australia as a spokesman, said it appeared the Prime Minister had significantly reduced the amount of sugar and fat in his diet and started exercising for a 'reasonably dramatic' result in just three weeks.  

In the first week of October, Mr Morrison's belly was visible under his button-up shirt as he greeted Holden V8 Supercar drivers at the Bathurst 1000

Prime Minister Scott Morrison (right) appeared to be a lot slimmer this week as he visited a gelato shop in Canberra

'Fundamentally, he's reduced his calories. For this to fundamentally happen, the first law of thermodynamics applies in that he's reduced his calorie intake and/or increased his energy output,' he told Daily Mail Australia on Thursday.

'In the time frame, just noting how busy a man he would be as Prime Minister, I'd expect that he's made some reasonable dietary changes.'

Mr Watson said the Prime Minister could help keep the weight off by packing a sandwich, fruit and some yogurt for lunch instead of buying a takeaway meat pie that packs more than double the calories.

Geraldine Georgeou, the chief executive of Designer Diets, said it would be difficult for the Prime Minister, who turned 50 this year, to keep his weight off.

'I would call it man-pause. There is definitely a hormone shift that occurs as we have the ageing male,' she told Daily Mail Australia.

ScoMo, as he is also known, was photographed at Parliament House in Canberra eating two sausages without any bread 

'It can be involving a lowering of testosterone. You also find a change in muscle mass and a reduction in muscle mass so as they get older, other signs will be male pattern balding and changing cholesterol. They might have fatty liver.' 

ScoMo was this month photographed at Parliament House in Canberra eating two sausages without any bread. 

Eating meat without any carbohydrates is consistent with the Atkins diet, which is based on high-protein food intake with lots of meat.

Ms Georgeou said the Atkins approach to eating would constitute an unbalanced diet.

'I have seen patients over my experience that do eat a high-protein diet and they do seem to have a short-term weight loss but they're not looking after their whole health,' she said.

Designer Diets chief executive Geraldine Georgeou advised the Prime Minister (pictured on October 7) to steer clear of the Atkins diet, under which carbohydrates are shunned and meat is embraced

'Bowel health can be compromised, and I think it's really important (that) it's all about balance.'

Ms Georgeou advised the Prime Minister to avoid having meat without any carbohydrates in future.

'With Scott, I would ask him to eat a lean, sausage, wholegrain sandwich, with a little bit of tomato sauce,' she said.

'It's much more fulfilling, sustainable and long-lasting energy, I would say. Turn it into a sanga.'

Tubby PMs are a rarity in Australia, with overweight former Labor leader Kim Beazley never making it to The Lodge, after losing the 1998 and 2001 elections.

As a man over 50, Scott Morrison faces a fall in testosterone levels, a drop in muscle mass, and male-pattern balding, a leading dietitian said

He even lost weight after returning to the Labor leadership in 2005 only to be overthrown in Opposition by a skinnier former diplomat in Kevin Rudd at the end of 2006. 

Rudd prevailed at the polls a year later against Prime Minister John Howard, who was found of a morning power walk.

Labor's current leader Bill Shorten has taken to jogging and has no longer has man boobs, which means the next federal election due by May 2019 is set to be a contest between two leaders who have shed the kilos. 

As Treasurer in April, Mr Morrison used a weight-loss metaphor to describe how the government would aim to return the budget to surplus in 2019 for the first time since 2007. 

'Australians are continuing to make sure that their belts are tightened and the government is no different so there will be a Budget that lives within its means,' he told reporters in Canberra.

As an Opposition frontbencher under Tony Abbott, Mr Morrison used to criticise Labor's inability to return the budget to surplus using the phrase, 'Sweet moment on the lips, forever on the hips.'

Labor's current leader Bill Shorten has taken to jogging and no longer has man boobs, which means the next federal election due by May 2019 is set to be a contest between two leaders who have shed the kilos

As prime minister in 2005, John Howard (pictured) was found of a morning power walk and twice defeated Labor leader Kim Beazley, who was overweight

That was before the then shadow treasuer Joe Hockey in 2013 opted for gastric sleeve surgery to reduce his stomach size by 20 per cent, as his boss Tony Abbott completed triathlons. 

ScoMo's predecessor as prime minister Malcolm Turnbull lost 14kg through a Chinese herbal tea diet in 2011, four years before he overthrew his ultra-fit leadership rival Tony Abbott. 

Mr Morrison was hailed as a role model for losing weight in a more conventional way. 

'To have our Prime Minister looking the way he does is a very strong role model for engagement in health strategies,' accredited practising dietitian Trent Watson said.

Scott Morrison (pictured at Parliament House in mid-October) has been hailed as a role model for losing weight in a more conventional way than adopting a crash diet

'They're going to bring positive outcomes, not only for him but hopefully in the way he executes his job as well.'

The Newcastle-based dietitian with 20 years' experience is just hoping the government introduces a tax on sugary and fatty foods, as the United Kingdom and Mexico have done, in a bid to combat obesity, diabetes, heart disease and cholesterol problems.

'They're policy decisions that would have a huge bearing. Demonstrated evidence in other countries where they have been brought in is the consumption of high-sugar and high-fat foods has been reduced,' he said, expressing a personal opinion.

The Prime Minister's office has confirmed ScoMo has changed his diet but they declined to elaborate or confirm how much weight he had lost.