The number of multi-billion dollar investable projects is at historic highs, reflecting investor confidence in the long-term outlook for the Australian economy, according to a report by Deloitte Access Economics.

"There are an unprecedented number of mega projects underway or in planning in Australia," the firm said.

"There are 14 projects that are each worth more than $10 billion currently in the Investment Monitor database. Of these, five have a cost exceeding $30 billion," it said.

The total value of investable projects grew by 7.5 per cent over the September quarter to $894.1 billion.

The value of projects that have been classed as under construction or committed to commence soon, or definite projects, is $406.8 billion, a 13.8 per cent increase compared to the June quarter, and a 51.3 per cent rise over the past year.

"The rise in their number in Australia's investment pipeline reflects the level of confidence that investors have in the longer term prospects of the global and Australian economies," Deloitte Access Economics said.

"Amid the extreme short-term volatility that we have seen on share markets and currency markets over the past couple of months, a continued focus on the longer term picture by investors is comforting," it said.

Fast growth of investable projects took place in the mining sector, which accounted for 34 per cent of all investment projects currently under construction.

"With mining projects dominating the investment pipeline it is no surprise then that resource rich Western Australia and Queensland are the states which are now way out in front in terms of definite projects," the firm said. 

In the absence of a large mining base, the other states are underperforming, partly due to a lack of investment in office, retail and tourism facilities.