The two major shopping malls, which form a consumer mecca stretching from Lonsdale to Bourke Street, share up to 44 tenants, including key brands like Armani Exchange, Arthur Galan, Hype DC and Scanlon & Theodore.

Calibre, Kookai, Sass & Bide, Platypus and Saba, among others, also have stores in both centres and will need to soon decide whether to keep two shops open in close proximity or shut one and concentrate on the more profitable space.

About 15 per cent of Melbourne Central's 301 tenants were enticed into setting up stores in the newly-minted Emporium when it opened in April last year.

The double-up of tenancies could prove a headache for both centres, but is likely to have a greater impact on the GPT-owned Melbourne Central where key shop leases come up for renewal over the next two years.

GPT chief investment officer Carmel Hourigan dismissed the concerns, saying only a very small number of retailers were unlikely to renew their leases in Melbourne Central.

"There are 44 retailers duplicated across the two centres and we are in active discussions with all of those, and the majority of these will renew their leases," she said.

Retail analyst with CLSA investment bank, Sholto Maconochie​, said strong demand from retailers wanting to trade in the CBD supported GPT's stance.

"There may be some stores that focus on the profitable ones but others will stay open if they're trading well," he said.

Both the Emporium and Melbourne Central were "highly productive" in terms of speciality sales.

Melbourne Central trades "above average" at 9,823 sales per square metre. By comparison, top trading monolith Chadstone achieves 15,412 speciality sales per square metre, while Westfield in Sydney leads the nation on 18,026 sales per square metre, Mr Maconochie said.

Colliers International puts the retail vacancy for the CBD retail core at 1.92 per cent.

Retail director Ben Tremellen said historic low vacancy rates were being underpinned by demand from international retailers.

At least 20 international retailers wanted to establish flagship stores in the city, including Mango, Banana Republic, Sephora, River Island, Match, Puma, Nike, Belstaff, Forever 21, Big Star, and True Religion, Mr Tremellen said.

Scanlon & Theodore founder Gary Theodore said he hoped to renew both leases.

"They've both worked fabulously for us. Both of them are very strong businesses. If we're able to do a deal suitable to both parties, we'll be staying," he said.

Melbourne Central's customer traffic had risen 8 per cent since Emporium opened, specialty sales were up 4.5 per cent and total centre sales 5 per cent, Ms Hourigan said.

The centre spent $1.5 million on revamping its Lonsdale Street frontage and also bolted on another small property at 202 Little Lonsdale Street in mid-2014 for $4.8 million.

GPT's 2014 accounts show Melbourne Central's value increased by more than $50 million over the year to $1,049 billion. It accounts for almost one quarter by value of GPT's $4.8 billion Australian retail portfolio, which includes 16 shopping centres, with 1,050,000 square metres of gross leasing area, and more than 3700 tenants.