With Prime Minister Najib Abdul Razak set to table a revised Budget 2016 in Parliament next week, former premier Dr Mahathir Mohamad has cautioned that the government could be in trouble if it continues to spend beyond its means.

"When you have less money, live within your means. If you live beyond your means, you will be in trouble," said Mahathir during a press conference at Proton Centre of Excellence in Subang Jaya today.

Mahathir said this when pressed for specific advice on what he would do if he was still the prime minister today, in light of current challenging economic times.

To an earlier question on what he would do if he was still the prime minister, Mahathir said: "I have replied to this question before. If I were the prime ministe now, I would resign."

"There are a lot of things to be done now but it would depend on many things," said Mahathir who is Proton chairman.

He had earlier witnessed the first firing of Proton's latest range of engines via live teleconferencing from the testing facilities in England.

At another Proton event last week, Mahathir had expressed his rare support for Najib's move to review Budget 2016.

Today, however, he noted that further revisions may need to be made as the review was based on crude oil price of US$30 per barrel.

"Today the price (of crude oil) has gone below US$30 and the review was based at the price of US$30 per barrel.

"So maybe now (Najib will) need to review (the budget) because the price is going down," said Mahathir, who cautioned that Malaysia must be ready to face a very difficult economic environment.

"There are a lot of things that we need to look at if we were to continue our development," he added.

'Probe papers not as tall as A-G'

On a separate matter, Mahathir also said that investigation papers on the RM2.6 billion donation scandal and SRC International are not as tall as Attorney-General Mohamed Apandi Ali and so the ongoing probe should not have taken so long.

"It's not so tall but a few hundred sheets tall, not as tall as the AG. So he wouldn't take that long to study them," said Mahathir when asked for comment on Apandi's statement today that the probe papers have been returned to the MACC for further clarification.

The investigation papers were submitted to the AG on Dec 31 last year.

Earlier this month, Mahathir had in an interview with Free Malaysia Today predicted that Apandi would ignore the MACC's report and that nothing conclusive would result from the investigations.

"But (now) I think he (Apandi) is very careful about studying the papers, so it will take quite a long time before he decides what to do," Mahathir said.

Najib has so far denied any wrongdoing in the cases investigated.