Queensland police and State Emergency Service volunteers are searching for a missing Brisbane father who hasn't been seen since he went for a motorcycle ride almost a week ago.

Key points: A search is underway for 66-year-old Sandgate father Siemon Mulder

A search is underway for 66-year-old Sandgate father Siemon Mulder He didn't return from a motorcycle ride to Esk on Monday

He didn't return from a motorcycle ride to Esk on Monday Police and SES volunteers are focusing on the Mt Glorious area but believe the bike could be anywhere in South East Queensland

Siemon Mulder left his home at Sandgate, north of Brisbane, on Monday morning and told his wife he intended to travel to Esk, north-west of Ipswich.

He was due back at midday on the same day, but never arrived.

SES crews have searched 18 of the 21 possible routes Mr Mulder may have taken while riding his blue Triumph Sprint ST motorcycle.

The area covers a 100-kilometre search radius involving roadways from Ferny Grove, through the Samford Valley to Esk and back through Dayboro.

Inspector Steve Flori urged the public to keep an eye out for the distinctive bike "anywhere in South East Queensland or elsewhere."

Police said Siemon Mulder was riding a blue Triumph Sprint ST motorcycle just like this one. ( ABC News: Dea Clark )

"We're focusing on the Mt Glorious area because that's the most prominent route," he said.

A local motorcycle club has also arranged a large-scale ride of the area this weekend.

Inspector Steve Flori says there are grave concerns for Mr Mulder's welfare.

"His family are distraught," he said.

Police say they have grave concerns for Siemon Mulder's welfare. ( Supplied: Crowdfunder )

"His wife and two children are very, very concerned — understandably — even to the point of engaging a private helicopter to do searches of their own."

"This is a 66-year-old normal man … and he's gone for a ride on his motorcycle for the day and hasn't come home.

"So we are very concerned about him. And we're very concerned that we need to find him quickly," Inspector Flori said.

Mr Mulder is described as approximately 180cm tall with a slim build, grey short hair and grey facial hair.

He was last seen wearing a black leather jacket, blue jeans and black helmet with a dark tinted visor.

Investigators are appealing for anyone who may have seen Mr Mulder or have dash cam footage of him travelling to or from Esk on Monday to contact police.