Investment will go towards increasing online presence in UK market
NAB, which posted a 24 per cent rise in first half profits, said the investment would be geared towards increasing its online presence in UK banking.
Its UK chief executive, David Thorburn, said he would be focusing his energy in growing the UK operation organically and was in no hurry to sell the UK operations.
However, NAB said it will consider "financially attractive" offers for its UK operations.
In a release accompanying annual results posted in Australia overnight, NAB said: "The United Kingdom financial services market is undergoing a period of change.
"The Group would consider opportunities in the United Kingdom that are financially attractive."
Profits from Clydesdale Bank rose 41 per cent to $288 million Australian dollars (£186.8 million) and analysts are now valuing Clydesdale Bank at around £2.27 billion ($3.5bn AUD)
Earnings from the UK banks rose 55 per cent in the year to the end of September, largely on easing of bad debt provisions.
NAB has reportedly been holding informal talks in recent months with two buyout funds to sell either all or part of Clydesdale as part of a longer-term plan to leave the UK market.
This led to credit rating agency Moody's downgrading Clydesdale Bank in September amid doubts about its Australian parent company's commitment to its UK operations.
Moody's cut its long-term bank deposit and senior debt rating down from A1 to A2 for both the Clydesdale Bank and Yorkshire Bank after NAB executive director for finance Mark Joiner hinted at a possible sale of the group's UK operations.
NAB recently rejected a £2 billion bid for its UK operations by private equity firm Sun Capital, which it said was too low.
NBNK Investments, a new banking venture set up by former Lloyds of London chairman Lord Levene had also expressed an interest in buying the Clydesdale and Yorkshire banks.
However NBNK Investments withdrew from negotiations after failing to agree a price.