An international investment management business has taken a big stand on Fletcher Building for a secret buyer, swooping on just over 5 per cent of the shares, indicating strong global interest in the business.

Ellerston Capital of Elizabeth St, Sydney wrote to the ASX on Friday, saying it has become a substantial holder.

Ellerston, with more than A$5 billion (NZ$5.27 billion) under management, now controls 5.13 per cent of the shares of the Penrose-headquartered builder, materials manufacturer, supplier and distributor on behalf of an unnamed party.

The Sydney-headquartered business' clients "include Government, industry and corporate superannuation funds, international funds as well as individual investors via leading investment administration platforms," Ellerston said.

"The shares reported in this form are owned by third-party accounts under the discretionary investment management of Ellerston Capital," the formal notice signed by Ellerston's company secretary Ian Kelly said.

"None of Ellerston Capital nor any of its shareholders or any of their associates hold a direct interest in the shares disclosed in this form," Kelly wrote.

"We attach our notice of becoming a substantial holder in respect of Fletcher Building."

Ellerston's 5.13 per cent stake amounted to 35,786,9434 ordinary shares, the notice showed.

Comment has been sought from Fletcher this morning on the big stand but not yet received.

Once any business buys more than 5 per cent of a company, a notice must be lodged with the stock exchange.

But whether Ellerston has bought on behalf of giant Australian business Wesfarmers remains unknown.

On Friday, Fletcher rejected speculation about Wesfarmers, which owns the Bunnings and Kmart businesses here.

"Fletcher Building noted today reports from various media outlets, stating that Wesfarmers has been accumulating Fletcher Building shares and now holds 3 per cent to 4 per cent of its shares," Fletcher said last week.

"The company has no knowledge of Wesfarmers owning Fletcher Building shares, therefore it can neither confirm nor deny the report," it said.

Fletcher shares were closing on Friday's NZX close around $6.34, a big rise on the $5.84 they had been at earlier last week.

If Ellerston bought at the low of $5.84 a share, it could have paid around NZ$208.9m.

Fletcher is dual-listed on the ASX and NZX.

Ellerston's ordinary shares come with voting rights, giving the business which controls the stake some say in the troubled Fletcher Building business, which said earlier this year it expected to lose nearly $1b in operating revenue from difficult construction jobs in New Zealand.