ExxonMobil Australia has announced an investment worth tens of millions of dollars in its Altona refinery in Melbourne's west.

The announcement is a vote of confidence for business investment in Victoria and the state's AAA economy rating.

The refinery is one of Victoria's key refinery plants, processing up to 13 million litres of crude oil daily into petrol, diesel, aviation fuel, LPG and bitumen.  It also employs around 350 of the company's 1,750 Victorian employees.

Production will increase as a result of the investment from 80,000 to 90,000 barrels per day, including diesel and jet fuel.  The refinery's crude oil plant will also be upgraded.

Improvements in energy efficiency from the upgrade will also enable the recovery of an additional three megawatts of waste heat.

This investment is in addition to the A$370 million ExxonMobil has already invested in maintenance and improvements to the refinery. The company's contribution to the Victorian economy is currently estimated at A$270 million annually.

This is the second major investment ExxonMobil has announced this year in its Victorian operations, commencing works on the A$400 million Longford pipeline replacement project in February.

ExxonMobil is a multinational American oil and gas corporation, ranked eighth in the world for company revenue in 2015.  Esso and Mobil form a part of ExxonMobil's operations in Australia.