Machell will report to Aviva group chief executive Andrew Moss.
Aviva Australia chief executive Allan Griffiths will now be responsible for driving the company's growth in Taiwan and Malaysia.
"As we build the business in the Asia Pacific region, Australia is going to form a strong, mature role," Griffiths said.
"We'll be passing on things we've learnt [to Malaysia and Thailand] and hopefully they won't make the same mistakes."
Griffiths will coach and mentor the Malaysian and Thai Aviva chief executives, Chris Knight and Ho Ming Heng respectively.
Each business will continue to run separately. The enterprises recently established joint ventures with local banks.
"I've taken it on because business in Australia is going very well," Griffiths said.
"We've got a good strong management team in place, so I'm quite comfortable to leave it to run itself for a few days a month."
As well as international appointments, two Aviva Australia executives have been promoted. Chief financial officer Craig Brackenrig has become Asia Pacific finance director.
Stepping into Brackenrig's former role is former group director of product, marketing and public relations Rob Donaghy.
The Asia Pacific region comprises Australia, China, India, Sri Lanka, Hong Kong, the Middle East, Singapore and Taiwan.
Aviva sells life assurance, investment and general insurance in the UK as Norwich Union and trades as Aviva in 25 markets globally.
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