But he said the market's history was made "somewhat irrelevant" by the low interest rate environment of recent years.
"You can't look at it and say it's expensive without contemplating that interest rates are one third of what they were when you had an OCR [official cash rate] of 6 per cent and home loan rates of 9 or 10 per cent," Williams said. "That's a completely different reference point, so I'd say the market valuation point is fine ... and within the market there are still opportunities."
Investors needed to be wary, however.
With a price-to-earnings multiple of around 6 times, Williams said Air New Zealand, for example, was one of the cheapest stocks on the market. But he said there was risk of intense competition eroding the airline's earnings base.
"Not a week goes by when there's not a new competitor announced," Williams said. "They've done an exceptionally good job of positioning themselves against that so maybe [Air New Zealand shares] are still cheap enough to invest in ... but we're cautious."
Meanwhile, he said Nikko had a positive view on the upcoming float of chicken producer Tegel, which will list on the Australian and New Zealand stock exchanges on May 3.
"We've done quite a bit of work on it," Williams said. "It's a very good company, with a good demand profile. It's simply going to come down to price."
Tegel shares have been priced in an indicative range of $1.55 to $2.50, giving the company a market value of up to $636 million. The deal, which will raise up to $344.4 million, gives Tegel an indicative price-to-earnings multiple of 12.7 to 14.7 times. Final pricing will take place through a bookbuild on April 18 and 19.
WATCH: Stuart Williams discusses the market's strong run
Strong run
• NZX closed at 6755.22 last night, having gained 6.8% this year
• Up 13.8% from the low it reached on February 12