At 31 March, IW&I had AUM of £45.5bn, whereas asset growth and net inflows have grown by £700m over the period.
In terms of the asset management business, operating profit increased by 16.6% over the period, from £70.6m to £82.3m.
Following net inflows of £1.1bn over the six months and a recovery in equity markets total AUM grew from £75.7bn in March to £89.8bn at 30 September.
The division also benefited from higher performance fees.
Stephen Koseff, chief executive of Investec, said in a market statement: “These results show the long-term strength and diversity of our business, delivering sustainable recurring client-driven income streams amidst high levels of macro uncertainty.”
The group cited Brexit, the US election result and the economic, political and social volatility still present in South Africa as the key macro challenges causing uncertainty.
Yet Investec said with its operational and geographical diversity, supporting a recurring revenue base it remained “resilient”.
Koseff added: “We are pleased with the growth in net interest income and fees and commissions, as well as good fund management inflows. We continue to invest with confidence in our business to build even stronger franchises."
Specialist banking posted a 7.1% decline in operating profits from £229.2m to £212.8m.
While it reported good levels of client activity and support of the franchise, its numbers were hit by a fall in investment income.
With cash and near-cash on the balance sheet of £13.1m, Investec Group will grow its dividend by 5.3%, from 9.5p last year to 10p per ordinary share.