They are mostly buying Apple, Facebook, Amazon and Microsoft.

Figures from Nabtrade, the sharebroking arm of the big bank, shows all age groups are favouring the tech companies that they know and whose products they use.

Tech stocks are potentially more rewarding than companies operating in other sectors of the economy, but also riskier.

Many start-ups and even those that have been going for a while, particularly in the tech space, often don't generate profits.

The Nabtrade figures show investors are mainly interested the mature techs with big profits. It was different in the late 1990s, the last time US-listed tech stocks had a huge run-up in their share prices.

US-based analysts put out notes to their clients just before it all came crashing down in early 2000 with the "tech wreck" saying how earnings didn't matter any more because it was all about market share.

How to value tech stocks

A share price should be related to the present value of future earnings, but there are shortcomings in applying the classic measure of value to innovative companies.

That measure is the price-to-earnings (PE) ratio, which is the share price divided by the earnings per share over 12 months.

A share price of $60 with earnings per share of $3 will have a PE of 20 times.

A "growth" stock, those whose earnings are expected to grow much faster than economic growth, tend to have shares trading at higher PEs.

Andrew Macken, portfolio manager at Montgomery Global, says the PE ratio is a rule of thumb for "business as usual".

In his opinion "it completely falls apart" when it comes to high-growth businesses.

Marcus Padley, Money columnist and author of the sharemarket newsletter Marcus Today, favours the PEG ratio for high-growth companies.

It is the PE divided by the expected annual earnings growth. If a company is on a PE of 10 times and its expected earnings growth is10 per cent, the PEG ratio is one.

A PEG ratio under one indicates the market is not fully pricing-in the growth in earnings.

Facebook shares are trading on a PE of about 40 times, which seems expensive. But it is on a PEG of about 0.5 times, which makes it cheap.

It implies the share price has not been bid-up fully to reflect the expected annual earnings growth.

Macken, who owns Facebook, doesn't regard Facebook as expensive; though, he understands why Australian investors might think otherwise, given many of the big Australian companies are on PEs of the order of 15 times.

Macken doesn't own Google (listed parent Alphabet), which he says is a "wonderful business" but just too expensive. And he doesn't own Netflix as it's unclear if it will ever make money.

Australian shares

Australia's big banks and Telstra and many other giants of the local market are often mature businesses.

They are much lower-growth than the big name tech stocks, but they generally pay much higher levels of tax-advantaged dividends.

Overseas-listed companies, particularly US-listed, tend to plough earnings back into growing the company and reward investors though higher share prices.

Those Australian investors ploughing into US tech stocks probably feel comfortable with the risk.

The Nabtrade figures show all age groups, while increasing their exposure to US tech stocks, are sticking to their home-grown favourites such as the big banks with their fully-franked dividends.

And there are not the risks with the techs that existed last time around.

Just before the tech wreck, shares listed on the Nasdaq Composite were trading on a PE of about 75 times.

Now, it is about 25 times which, for the high-growth tech sector, seems reasonable.

Macken says earnings of the Nasdaq-listed companies are about five times higher today, on average, than at the time of the tech-wreck.

He says the risk is with each tech stock, rather than the sector overall.

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